Toast the Bread: Preheat the oven to 350°F. Slice the baguette into ½-inch thick pieces. Brush each piece with melted butter and rub with smashed garlic. Place on a baking sheet and toast for 5-7 minutes until golden brown. Set aside.
1 loaf French baguette, 2 tablespoons butter, 2 cloves garlic
Make the Balsamic Reduction: Pour balsamic vinegar into a small saucepan and bring to a boil. Reduce heat and let it simmer until it thickens and reduces by half. It should coat the back of a spoon. Turn off the heat and set aside to cool.
1 cup balsamic vinegar
Prepare the Blue Cheese Sauce: In a small saucepan, combine the blue cheese, milk, and one smashed garlic clove. Season with salt and pepper. Heat over medium, stirring frequently, until the cheese is fully melted and smooth. Adjust milk for desired consistency. Turn off the heat and set aside.
1 cup crumbled blue cheese, Salt and pepper, 2 tablespoons milk
Cook the Steak: Heat a greased grill pan or skillet over medium-high heat. Season the steak pieces with salt and pepper. Sear for 1-2 minutes per side for rare to medium-rare. Remove from heat and let rest for 2-3 minutes before assembling.
1 pound sirloin steak
Assemble the Crostini: Spread the blue cheese sauce on each toasted baguette slice. Top with a piece of steak, drizzle with balsamic reduction, and garnish with fresh herbs. Serve immediately.
Fresh herbs for garnish
Make-Ahead Tip: The balsamic reduction and blue cheese sauce can be made ahead of time. Store in airtight containers in the refrigerator for up to 2 days. Gently reheat the sauce before assembling.