01 -
Spray a 9 x 13 baking pan lightly with non-stick spray. Spread the bread cubes evenly over the bottom.
02 -
Scatter the shredded cheese over the bread layer. Add the pieces of bacon on top.
03 -
Crack the eggs into a mixing bowl and whisk until smooth. Stir in the milk, ground mustard, and salt, if you're using it. Pour the egg mixture evenly over everything in the pan.
04 -
Cover the pan and leave it in the fridge overnight so the flavors soak in.
05 -
While you're heating the oven to 350°F, take the casserole out of the fridge. Bake it uncovered for 30–45 minutes, checking that the center is firm.