Preheat the oven to 400 degrees F (200 degrees C).
Slice the loaf of French bread HORIZONTALLY into 5 equal layers. Spread bottom layer with horseradish, top with roast beef. Place the next slice of bread over beef, spread with mustard and top with ham and Swiss cheese. Add next slice of bread, spread with mayonnaise and top with turkey and cheese. Add the next slice of bread, spread with more mayonnaise top with tomato and onion. Cover with top. Brush melted butter over top and sides of loaf (if desired). Place on a baking sheet, loosely tent with heavy duty foil.
Bake for 15 to 20 minutes in the preheated oven, or until cheese is melted and sandwich is heated through. Carefully slice into 6 slices and serve.
