Lightly grease an oven-proof baking dish (about 8x8x2). Put 4 slices of bread next to each other on the bottom of the prepared dish. Put a slice of cheese on top of each slice. Sprinkle bacon bits over top of cheese. Top with the other 4 slices of bread. In a small mixing bowl, beat the eggs until foamy. Stir in the milk, salt, pepper, onion, mustard. Pour the egg mixture over the bread stacks. Cover the baking dish and let it sit on the counter for at least 1 1/2 hours or put it into the refrigerator for 12 hours. When ready to bake, preheat oven to 325F. Remove cover from pan and bake for about one hour. Let dish cool for about 5 minutes and serve.