Creamy Leek and Potato Gratin with Gruyere and Thyme

The first forkful should slide through tender potato layers instead of pushing back with a firm center. I cut the Yukon Golds to a consistent 1/8 inch for this creamy…

By Julia • November 1, 2026 • Reading time: 9 min

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The first forkful should slide through tender potato layers instead of pushing back with a firm center. I cut the Yukon Golds to a consistent 1/8 inch for this creamy leek and potato gratin; even slices cook at the same pace, which matters in a casserole this deep. The leeks soften into a mellow layer between the potatoes, while Gruyere gives the top a nutty, savory edge.

Warming the cream mixture until it’s steaming before it goes over the potatoes means the gratin starts baking with warm liquid rather than a cold pour. Keep it below a boil on the stove. That gentle approach lets the thyme come through without taking over, and the finished dish has a soft, spoonable middle under a golden cheese top.

Why the Gruyere Goes on After the Potatoes Are Tender

The potatoes and the cheese need different things from the oven. While the dish is covered, trapped steam helps soften the potato slices all the way through. If the cheese went on at the start, it would spend that entire bake exposed to heat and could turn dark before the center potatoes were tender. Giving the potatoes their own covered bake solves that timing problem.

Keep the foil snug over the baking dish so steam stays inside instead of escaping around the edges. When you uncover the gratin, it may look pale and creamy; that’s expected. The Gruyere gets its own uncovered stretch to melt, bubble, and brown while the potatoes are already cooked through. Judge the first stage by the center, not by how tender the outer slices look: a knife should slide easily into the middle before you add the cheese.

The Ingredients That Give This Gratin Its Creamy Layers

• Leeks — Use the white and light green parts, thinly sliced and thoroughly rinsed. They become soft and mild in the skillet; there isn’t a close substitute that gives the gratin the same gentle leek flavor. Standard grocery-store leeks work well, so there’s no need to seek out a specialty variety.
• Yukon Gold potatoes — Their creamy texture suits a layered bake, and they hold their shape better than a potato that breaks down quickly. Peel them and slice them evenly; no substitute will give quite the same balance of tender texture and distinct layers.
• Heavy cream and whole milk — Together they make a rich but pourable liquid that moves through the potato layers. Regular grocery-store dairy is fine here. Replacing one with more of the other changes the balance, so I keep both in the amounts given rather than trying to make one stand in for the other.
• Gruyere — This is the prominent flavor on top, with a nutty savoriness that works especially well against the mild potatoes and leeks. Shred it evenly so it covers the dish without leaving bare patches. A different cheese can melt, but it won’t give the same flavor.
• Garlic, fresh thyme, and nutmeg — Garlic gives the cream a savory base, thyme brings a light herbal note, and just a touch of nutmeg adds warmth behind the dairy. Strip the thyme leaves from their stems before measuring them; fresh leaves blend into the cream more easily than woody stem pieces.

Build the Gratin in Two Bakes, Not One

Prepare the Baking Dish

Heat the oven to 375°F. Grease a 9×13-inch baking dish with 1 teaspoon of the butter, spreading it across the bottom and sides. The butter is divided between the dish and the skillet, so keep the remaining portion for the leeks.

Soften the Leeks in Butter

Melt the remaining 2 teaspoons of butter in a large skillet over medium heat. Add the thoroughly rinsed leeks and cook for 6 to 8 minutes, stirring occasionally. They’re ready when they’ve softened; keep the heat at medium so they cook gently rather than scorching against the skillet.

Warm the Cream Mixture Until Steaming

Combine the heavy cream, whole milk, minced garlic, thyme leaves, salt, black pepper, and nutmeg in a medium saucepan. Warm the mixture over medium-low heat until it’s steaming, then remove it from the heat. Don’t let it boil; steaming is the cue to take it off the burner.

Layer the Potatoes and Cover Tightly

Arrange half of the potatoes in the prepared baking dish, spread the softened leeks over them, then layer on the remaining potatoes. Pour the warm cream mixture evenly over the top so it can settle between the slices. Cover the dish tightly with foil and bake for 45 to 55 minutes. A knife should slide easily into the center potatoes before you move on.

Brown the Gruyere and Let the Gratin Rest

Remove the foil and sprinkle the Gruyere evenly over the potatoes. Bake uncovered for 20 to 25 minutes, until the cheese is golden and bubbling. The center has already had its covered bake, so this final stage is about melting and browning the topping. Let the gratin rest for 15 minutes before serving so the creamy layers can settle.

Small Changes That Keep the Gratin Balanced

Questions That Come Up While Making This Gratin

Creamy Leek and Potato Gratin with Gruyere and Thyme

Creamy leek and potato gratin layers tender Yukon Gold potatoes and softened leeks in a thyme-scented cream sauce. It bakes under foil until tender, then uncovered until the Gruyere topping is golden and bubbling.

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Prep Time 25 minutes mins

Cook Time 1 hour hr 20 minutes mins

resting 15 minutes mins

Total Time 2 hours hrs

Servings: 8 servings

Course: Side Dish

Cuisine: French

Calories: 375

Ingredients Equipment Method Notes

Ingredients

  

For the gratin

• 1 tbsp unsalted butter divided
• 2 leeks medium; white and light green parts only, thinly sliced and thoroughly rinsed
• 2 lb Yukon Gold potatoes peeled and sliced 1/8 inch thick
• 1.5 cup heavy cream
• 1 cup whole milk
• 2 cloves garlic minced
• 1 tsp fresh thyme leaves
• 1 tsp kosher salt
• 0.25 tsp black pepper
• 0.125 tsp ground nutmeg

For the topping

• 6 oz Gruyere cheese shredded (about 1 1/2 cups)

Equipment

• 1 9x13 inch baking dish

Method

 

Prepare the gratin

1. Heat the oven to 375°F.
2. Grease a 9x13-inch baking dish with 1 teaspoon of the butter.
3. Melt the remaining 2 teaspoons butter in a large skillet over medium heat.
4. Add the leeks and cook for 6 to 8 minutes, stirring occasionally, until softened.
5. In a medium saucepan, combine the heavy cream, milk, garlic, thyme, salt, pepper, and nutmeg.
6. Warm the cream mixture over medium-low heat until steaming, then remove from the heat.
7. Arrange half the potatoes in the prepared dish, spread the leeks over them, and layer the remaining potatoes on top.
8. Pour the warm cream mixture evenly over the potatoes.
9. Cover the dish tightly with foil and bake for 45 to 55 minutes, until a knife slides easily into the center potatoes.

Finish and bake

1. Remove the foil and sprinkle the Gruyere evenly over the top.
2. Bake uncovered for 20 to 25 minutes, until the cheese is golden and bubbling.
3. Let the gratin rest for 15 minutes before serving.

Notes

For even cooking, slice the potatoes uniformly and rinse the leeks thoroughly. Refrigerate leftovers for 3 to 4 days; freezing is not recommended because the cream sauce may separate. For a lighter option, replace some of the heavy cream with whole milk.