Preheat oven to 375°F. Slice the baguette into ½-inch rounds, brush with olive oil, and bake until golden and crisp, about 8–10 minutes.
Heat olive oil in a skillet over medium heat. Add sliced onions and cook, stirring frequently, until caramelized, about 20 minutes. Set aside.
Season steak with salt and pepper. Sear in a hot skillet for 3–4 minutes per side. Let rest for 10 minutes, then slice thinly against the grain.
Mix horseradish with sour cream and a pinch of salt. Adjust to taste for spice level.
Assemble crostini: Spread horseradish sauce on each toast, top with steak slices and caramelized onions. Garnish with fresh herbs if desired.
