Preheat the oven to 400°F.

Cut the French bread in half lengthwise. In a small bowl, combine the butter, garlic, Parmesan and parsley. Mix well. Spread half of the butter mixture on each half of the cut side of the bread. Put the bread buttered sides together and wrap the loaf in foil. Place in the oven for 15 minutes.

Meanwhile, melt the butter in a large skillet over medium-high. Add the mushrooms and onions and cook until softened, 10-15 minutes. Add the garlic and cook until fragrant, about 60 seconds, then add the Worcestershire sauce. Turn off the heat.

When the bread is warmed through, remove and place on a large baking sheet, buttered sides facing up. Put half of the roast beef on each side, followed by half of the mushroom mixture on each side. Top with the cheese. Return to the oven and bake until the cheese is melted and bubbly, 3-5 minutes. Cut each piece into 3 pieces to serve.