Sauté onions and mushrooms in the leftover potato frying grease. Lightly toast the bread. Slather it with half the gravy. Layer bacon, then potatoes, then veg, then more gravy, then cheese. Melt cheese however suits. However tempted you may be to pick this up like a slice of pizza, I strongly advise approaching it with a fork, unless you are dead sure of the structural integrity of your bread.