*Use white, sourdough, whole wheat, or brioche loaves, or about 2 long baguettes, sliced about 1-inch thick; you'll need about 40 slices of baguette.
Heavily butter a 9 x 13-inch baking pan. Spread half of the bread with the mustard (if you're using it). Place them into the greased baking dish, mustard-side up. Top with the ham, tomato, onion, and about three-quarters of the cheese. Top with the remain
In a large mixing bowl, beat together the eggs, milk, salt and pepper. Pour this mixture slowly over the bread, allowing it to seep into the cracks and crevices. Sprinkle with the remaining cheese, cover the dish, and refrigerate it for several hours, or
About 1 hour before serving, preheat the oven to 400°F. Uncover the baking dish, and bake the french toast for 40 to 45 minutes, until the top is golden brown and the eggs seem set. Check the dish after 30 minutes, and cover it lightly with aluminum foil