Preheat your oven to 375°F (190°C). Slice the baguette into 1/2-inch pieces and arrange on a baking sheet. Season the steak with salt and pepper.

In a food processor, combine parsley, garlic, olive oil, and red wine vinegar. Blend until smooth and adjust seasoning.

Melt butter in a skillet over low heat, add sliced onions, and cook for about 20 minutes until golden brown.

Heat a grill pan over medium-high heat, add steak, and sear for 4-5 minutes on each side for medium-rare. Let rest for 5 minutes before slicing.

Mix softened butter with minced garlic and a pinch of salt. Spread this on the toasted baguette slices.

Brush baguette slices with garlic butter and toast in the oven for 5-7 minutes until golden brown.

Layer a slice of steak, a spoonful of chimichurri, and caramelized onions on each toasted baguette slice.