Step 1

Preheat oven to 350°F. Lightly spray a 9 x 13 inch glass baking dish with cooking spray.

Step 2

Brown sausage in a large frying pan over medium heat, breaking up clumps as it browns. Transfer cooked sausage to a paper-towel lined plate to drain fat. Spread sausage on the bottom of the baking dish.

Step 3

Wilt spinach in the same pan over medium heat, covered, tossing occasionally, for a couple of minutes. Arrange spinach over the sausage layer. Sprinkle on 1 cup of cheese. Add the bread cubes in an even layer.

Step 4

In a large bowl, whisk together eggs, milk, salt, pepper and dry mustard powder. Pour egg mixture evenly over bread. Press bread down a little, if needed, so that all of it is soaked with the egg mixture. Sprinkle remaining 1 cup cheese over the top. At this point the egg casserole can be be covered and refrigerated for up to 24 hours (see note).

Step 5

Bake for 45 to 50 minutes or until set. To check for doneness, insert a table knife in the center of the casserole. It should come out clean. Remove from oven and let stand for at least five minutes before serving.