11/2c. milk (use part half-and-half for a richer dish)
Arrange 4 half slices of bread to fill bottom of buttered 8-inch square pan. Sprinkle bread with half the cheese. Top cheese with a vegetable or two (such as frozen chopped broccoli, cut green beans, cut asparagus, chopped spinach, or mixed vegetables -- or shredded carrot, finely chopped celery or a tablespoon or two of minced onion). Atop vegetables, add leftover cooked meat or a canned meat or fish. (Other protein sources: tuna, salmon, shrimp; diced sandwich meats such as honey ham; leftover roast beef, pork or chicken).
Cover all with the remaining 4 buttered half slices of bread. Distribute remaining cheese evenly over bread. Beat together eggs, milk and dry mustard; pour over layers. Cover and refrigerate until ready to bake.