In a large skillet brown ground beef and onions together. Drain any fat away. Add garlic and cook for 2 minutes until fragrant.
Meanwhile in a separate pan, brown mushrooms. Season with salt and pepper.
Mix together cooked mushrooms and ground beef mixture. Season with salt and pepper to taste, stir in Worcestershire sauce. Remove from heat and stir in sour cream.
Fill the bottom loaf of bread with meat mixture. Sprinkle with cheese. Cover with the other half other bread. Bake for 10-20 minutes until cheese is melted. Cut into slices to serve.