Preheat your oven to 350°F (175°C) and prepare a large baking sheet by lining it with parchment paper.

In a skillet over medium-high heat, melt 2 tablespoons of unsalted butter, then add the thinly sliced ribeye steak, seasoning with kosher salt and black pepper. Cook until no longer pink.

Stir in 2 tablespoons of Worcestershire sauce and cook for another 2-3 minutes, then remove from heat.

Add the remaining 2 tablespoons of unsalted butter to the skillet, then toss in the sliced white onion. Cook over medium-low heat for about 10 minutes until caramelized.

Remove the onions and sauté the green and red bell peppers in the same skillet until softened.

On the baking sheet, slice the French bread in half lengthwise and place cut-side up. Mix mayonnaise with horseradish sauce and spread over the bread.

Layer cooked ribeye steak, then caramelized onions and sautéed bell peppers over the mayonnaise spread.

Sprinkle shredded provolone cheese generously over the layered fillings.

Bake in the preheated oven for about 15 minutes, until cheese is melted and bubbly.

Remove from oven, let cool briefly, garnish with parsley, and slice for serving.