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Steak Crostini with Horseradish Sauce & Caramelized Onions

Steak crostini with horseradish sauce and caramelized onions on a wooden board.
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This impressive appetizer transforms a single flank steak into dozens of elegant bites. Featuring crispy garlic toasts, tender sliced steak, sweet caramelized onions, and a creamy, zesty horseradish sauce, it's the ultimate party starter that feels gourmet yet is built on simple techniques.

Ingredients

Scale
  • For the Caramelized Onions:
  • 2 large yellow onions, thinly sliced
  • 2 tablespoons unsalted butter
  • 1 tablespoon olive oil
  • 1/2 teaspoon kosher salt
  • 1/4 teaspoon black pepper
  • 1 teaspoon brown sugar (optional)
  • For the Horseradish Sauce:
  • 1/2 cup sour cream
  • 1/4 cup mayonnaise
  • 2-3 tablespoons prepared horseradish
  • 1 tablespoon fresh chives, finely chopped
  • 1 teaspoon fresh lemon juice
  • 1/4 teaspoon garlic powder
  • Salt and pepper to taste
  • For the Steak & Crostini:
  • 1 lb flank steak or skirt steak
  • 1 tablespoon olive oil
  • 1 teaspoon kosher salt
  • 1/2 teaspoon black pepper
  • 1/2 teaspoon garlic powder
  • 1 French baguette, sliced into 1/2-inch pieces
  • 2 tablespoons olive oil, for brushing
  • Flaky sea salt and fresh chives, for garnish

Instructions

  1. Make the Caramelized Onions: In a large skillet over medium-low heat, melt butter with 1 tbsp olive oil. Add sliced onions and 1/2 tsp kosher salt. Cook, stirring occasionally, for 40-50 minutes until deeply golden brown. Stir in brown sugar (if using) in the last 10 minutes. Set aside to cool.
  2. Make the Horseradish Sauce: In a small bowl, whisk together sour cream, mayonnaise, horseradish, chives, lemon juice, and garlic powder. Season with salt and pepper. Cover and refrigerate.
  3. Cook the Steak: Pat steak dry. Rub with 1 tbsp olive oil, then season all over with 1 tsp salt, 1/2 tsp pepper, and 1/2 tsp garlic powder. Let sit at room temp for 15-20 min. Heat a heavy skillet over medium-high heat. Cook steak for 4-5 minutes per side for medium-rare. Transfer to a cutting board and let rest for 10 minutes.
  4. Prepare the Crostini: Preheat oven to 375°F (190°C). Arrange baguette slices on a baking sheet, brush lightly with olive oil, and bake for 8-10 minutes, flipping once, until golden and crisp. Let cool slightly.
  5. Assemble: Slice the rested steak thinly against the grain. Spread a thin layer of horseradish sauce on each crostini. Top with a small amount of caramelized onions, then 1-2 slices of steak. Garnish with flaky sea salt and fresh chives. Serve immediately.

Notes

Caramelized onions and horseradish sauce can be made up to 2 days ahead. For best texture, assemble crostini just before serving. Always slice flank steak against the grain for tenderness.

Nutrition

Find it online: https://www.stacksipsnack.com/steak-crostini-horseradish-sauce-caramelized-onions/