Preheat oven to 400°F (200°C). Slice baguette into ½-inch thick pieces and place on a baking sheet. Thinly slice steak against the grain.

In a food processor, combine fresh herbs, garlic, olive oil, red wine vinegar, salt, and pepper. Pulse until well blended but still textured.

Heat olive oil in a skillet over medium heat. Add sliced onions and sauté for about 20 minutes until soft and golden brown.

In another skillet, heat high heat and sear sliced steak for 3-5 minutes per side until browned. Let steak rest for 10 minutes.

Mix softened butter with minced garlic and a pinch of salt in a small bowl. Spread garlic butter over baguette slices before toasting.

Spread garlic butter on each baguette slice, top with seared steak, caramelized onions, and drizzle with chimichurri.

Arrange crostinis on a platter, garnish with herbs if desired, and serve warm for the best experience.