In a large oven-safe skillet, melt the butter over medium heat. Then add the onion and bell pepper. Cook, stirring frequently, until softened, about 4-5 minutes.
Add the steak and garlic to the skillet. Cook for 1 minute until the steak is warmed through and the garlic is fragrant.
Add the cream cheese, sour cream, and Worcestershire sauce to the skillet and stir until melted and smooth. Add the Monterey jack cheese, provolone cheese, and half the mozzarella cheese. Stir into the dip until melted. Top with the remaining mozzarella cheese.
Broil for 2 minutes or until the cheese on top is golden and the dip is bubbling.
Carefully remove the skillet from the oven. Top with chopped parsley and serve warm with baguette slices for dipping.
Notes
If you don't have an oven-safe skillet, you can transfer the dip to a square baking dish once the cheese has melted. Then top the dip with the remaining mozzarella and broil as directed.