Arrange crostini on prepared baking sheet. Brush crostini lightly with melted butter. Bake until lightly toasted, approximately 5 minutes.
Line another baking sheet with aluminum foil.
Heat a small nonstick sauté pan over medium-high heat.
Drizzle both sides of fillet with olive oil, and season with garlic powder, salt, and pepper. When pan is hot, sear fillet on all sides until brown and slightly charred. Place on prepared baking sheet, and place in oven to finish cooking until desired degree of doneness is achieved, 5 to 7 minutes for rare. Remove from oven, and let rest for 10 minutes.
Using a long sharp knife, slice beef thinly.
In a small bowl, combine mayonnaise and horseradish.
Spread a layer of mayonnaise mixture onto each crostini. Divide beef among crostini.
Garnish each crostini with a parsley leaf, if desired.
Recipe by TeaTime Magazine at https://teatimemagazine.com/beef-fillet-crostinis-recipe/