Step 1

Grease or spray a 913-inch (or similar size) casserole dish with nonstick spray.Arrange the bread in a single layer in the pan, then set it aside.

Step 2

In a large pan, cook bacon according to package directions on the heat. All but roughly 1 tablespoon of the fat should be drained. (Alternatively, drain everything and add 1 tablespoon olive oil to the following step.) Allow bacon to cool until it is safe to handle, then slice into bite-size pieces. Sprinkle over the bread.

Step 3

Add the onion to the oiled skillet. Cook for 4 minutes over medium heat until tender. Cook, stirring periodically until the spinach has wilted, about 2-3 minutes. Remove from the heat. Spoon equally over the bread/bacon, then top with the remaining cheese.

Step 4

In a large mixing bowl, combine the eggs, milk, salt, ground mustard, and pepper.

Step 5

Distribute equally over the cheese. Cover securely and place in the refrigerator for 1 hour or overnight (up to 12-16 hours).

Step 6

Allow the casserole to come to room temperature for 15 minutes after removing it from the refrigerator.

Step 7

Preheat the oven to 375 degrees Fahrenheit (191 degrees Celsius).

Step 8

Bake the casserole for 20 minutes, uncovered. Bake for 30-35 minutes, loosely covered with aluminum foil. The casserole is done when the sides are browned and the center is puffy–this normally takes 50-55 minutes altogether.

Step 9

Remove from the oven, sprinkle with desired garnish(es), and set aside for 5 minutes before serving.Refrigerate leftovers for up to 5 days, securely wrapped in plastic wrap. Microwave to reheat.

Step 10

Refrigerate leftovers for up to 5 days, securely wrapped in plastic wrap. Microwave to reheat.