Grease a 9×13-inch casserole dish (or similar size) or spray with nonstick spray.

Arrange bread in a single layer in the pan, then set pan aside.

Cook bacon according to package stovetop directions in a large skillet. Drain all but about 1 Tablespoon grease. (Or drain it all and add 1 Tablespoon olive oil for the next step.) Cool bacon until ready to handle, then chop into bite-size pieces. Sprinkle over bread.

Add onion to greased skillet. Cook on medium heat until soft, about 4 minutes. Add garlic and spinach and while stirring occasionally, cook until spinach has wilted, about 2-3 minutes. Remove from heat. Spoon evenly over the bread/bacon, then sprinkle all of the cheese on top.

Whisk eggs, milk, salt, ground mustard, and pepper together in a large bowl. Pour evenly over cheese. Cover tightly and refrigerate for 1 hour or overnight (up to 12-16 hours).

Remove casserole from the refrigerator and allow to sit out at room temperature for 15 minutes.

Preheat the oven to 375°F (191°C).

Bake casserole, uncovered, for 20 minutes. Loosely cover with aluminum foil and bake for 30-35 minutes. The casserole is done when browned on the sides and puffy in the center—usually takes 50-55 minutes total.

Remove from the oven, sprinkle optional garnish(es) on top if using, and allow to cool for 5 minutes before serving.

Cover leftovers tightly and store in the refrigerator for up to 5 days. Reheat in the microwave.