Fry ground beef, onion and celery until ground beef is cooked.
Add ketchup, prepared mustard and salt.
Layer 4 slices of toast, half of beef mixture and half cup of cheese in greased baking dish (this is the one I use).*
Repeat toast and beef layers.
Mix milk, egg, dry mustard and pepper and pour over everything.
Bake at 350° for 30 minutes.
Sprinkle remaining cheese on top and let sit for 5 minutes to melt cheese.
* I’ve used an 8×8 baking dish before, but you kind of need to really squish the bread in to fit. If you end up using an 8x8 dish, you could just tear the bread into pieces instead. That should work too!