In a large skillet, saute the ground beef and onion together until the ground beef's cooked through and the onion is soft. Strain the meat and onions to remove grease, then return to the skillet. Stir in the crumbled bacon and continue to cook the meat mixture for an additional minute or two.
Stir in the soup, sauces, and spices until evenly incorporated. Let the mixture simmer for 8-10 minutes, stirring occasionally.
While the mixture simmers, slice the top third off of bread, and pull out pieces of bread in the middle of the bottom of the loaf forming a trench to hollow out bottom half of loaf, being careful not to thin it out too much so it doesn't 'leak' when filled. Set aside the discarded bread-- it can be stored to use as stuffing, or in meatloaves and meatballs.
Spread the butter out over the bottom of the 'top' piece of the loaf.
Transfer the hollowed out bottom piece to a flat baking sheet and put it in the oven under the broiler just until it's lightly browned.
Remove the toasted loaf from the oven and fill it with the warm meat mixture. Spread the cheese slices out evenly overtop. It's ok if they overlap some!
Place buttered top next to it, face up, on the baking sheet. Transfer the loaf back into the oven. Bake at 350 degrees, checking occasionally so the top toasts but doesn't burn, for 10-15 minutes-- until the cheese is melted and the meat mixture's heated through.
Top the melted cheese with toppings of choice. We listed our favorites in the ingredients, but this meal is made for mixing it up, so get creative!
Spread a thin layer of mayo onto the bottom of the toasted top slice. Put the top on the loaf, and use a serrated knife to slice the prepared loaf into sandwiches, about 3-4" wide each, or smaller 'strips' for appetizer/party-sized slices.