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Blue Cheese Steak Crostini with Balsamic Caramelized Onions

Blue cheese steak crostini pairs thinly sliced sirloin with a creamy blue cheese spread and balsamic caramelized onions on crisp, golden baguette slices. Toasted in the oven and assembled just before serving, each bite combines savory steak, tangy cheese, and sweet onions.
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Prep Time 15 minutes
Cook Time 1 hour 8 minutes
Resting 3 minutes
Total Time 1 hour 26 minutes
Servings: 8 servings
Course: Appetizer
Cuisine: American
Calories: 410
Ingredients Equipment Method Notes

Ingredients
  

For the onions
  • 2 tbsp unsalted butter
  • 2 yellow onions medium, halved and thinly sliced
  • 1.5 tsp kosher salt divided
  • 2 tbsp balsamic vinegar
For the steak
  • 1 lb boneless sirloin steak 1-inch-thick
  • 0.5 tsp black pepper
  • 3 tbsp olive oil divided
For the crostini
  • 1 loaf (about 12 oz) baguette cut into 24 1/2-inch slices
For the blue cheese spread
  • 4 oz cream cheese softened
  • 4 oz blue cheese crumbled

Equipment

  • 1 12-inch skillet
  • 1 18x13-inch rimmed baking sheet

Method
 

Caramelize the onions
  1. Melt the butter in a 12-inch skillet over medium-low heat, then add the onions and 1/2 teaspoon of the salt. Cook, stirring every few minutes, for 35-45 minutes, until the onions are very soft and deeply golden.
  2. Stir in the balsamic vinegar and cook for 2-3 minutes, until glossy and nearly evaporated. Transfer the onions to a plate and wipe the skillet clean.
Cook the steak
  1. Pat the steak dry and season it with the remaining 1 teaspoon salt and the pepper. Heat 1 tablespoon of the olive oil in the skillet over medium-high heat, then cook the steak for about 4-5 minutes per side, checking with an instant-read thermometer until it reaches 145°F.
  2. Transfer the steak to a cutting board and let it rest for 3 minutes, then slice it thinly against the grain.
Toast the crostini
  1. Heat the oven to 400°F. Arrange the baguette slices on an 18x13-inch rimmed baking sheet, brush both sides with the remaining 2 tablespoons olive oil, and bake for 8-10 minutes, turning once halfway through, until crisp and golden.
Make the blue cheese spread
  1. Mash the cream cheese and blue cheese together in a medium bowl with a fork until combined.
Assemble the crostini
  1. Spread the blue cheese mixture over the toasted baguette slices, then top each with sliced steak and caramelized onions.

Notes

For the best texture, assemble the crostini shortly before serving so the baguette stays crisp. Refrigerate leftover components separately for up to 3 days; freezing is not recommended. Use a gluten-free baguette for a gluten-free option.