Prepare the chimichurri sauce by chopping parsley, cilantro, and garlic, mixing with red wine vinegar and olive oil. Caramelize onions by cooking sliced onions in olive oil for 20-25 minutes until golden brown.

Season steak with salt and pepper, let rest for 10 minutes. Preheat grill over medium-high heat, cook steak for 4-5 minutes per side until medium-rare. Rest for 5 minutes before slicing thinly.

Slice baguette into ½-inch pieces, preheat oven to 400°F. Brush slices with garlic butter and bake for 8-10 minutes until crispy.

Assemble crostini by spreading garlic butter on toasted baguette, layering sliced steak, chimichurri sauce, and caramelized onions.

Serve the assembled crostini warm on a platter for best taste and texture.