Heat oven to 350 degrees.
Toast your bread lightly and spray both 9x13 casserole pans with cooking spray.
As your bread finishes toasting, butter each piece both sides, and lay a layer in the bottom of your casserole pan. You may need to cut to make them fit. Cutting and puzzle-piecing together, Tetris-style is A-OK.
Once meat is browned, drain it, then stir in the chopped onion, celery, mustard, garlic (and any additional vegetables you desire) with chosen seasonings and saute a couple minutes until the onion is soft. Try not to eat from the spatula as you stir, I know it's tempting.
In a bowl, whisk together the egg, milk, and some seasonings, place it inside an easily pourable measuring cup and set aside.
(You can choose to follow the following simple trick of ensuring enough meat is in both casseroles, or ignore and do you're own thing.) Draw a "line" in the skillet of meat, dividing it by halving it evenly. Draw another line across, separating the cooked meat mixture into four equal sections. Each section will now become a layer in the two casserole dishes. (See how that works! Math! Fractions! At work!)
Place a layer of meat on top of the buttered toast, and a layer of cheese. Season a little.
Place a new layer of buttered toast over top, followed by a layer of beef, then cheese again. Season a little again. Do not eat anything yet, don't spoil it!
Pour the egg milk mixture over top both casseroles, being careful to pay attention to how much you are using on each. (Here's why it's important to pour from the measuring cup, and why I mentioned potentially needing more egg/milk mixture.)
If, after pouring, it seems to dry and not covered enough, feel free to beat at additional egg and 3/4 cup of milk to pour until you feel it's completely covered nicely.
This egg/milk mixture is the "glue" to this casserole that keeps it together, and helps make it super-dee-duper yummy. I like to make sure all the crusts are covered nicely, and that it seems to set all the ingredients together well.
Once poured, spray your foil with cooking spray (so it won't stick), cover, and place it into the oven for at least 40 minutes to cook. Turn your oven light on so you can peek, especially if your casserole dish is see-through like mine.