Grease a 9x13-inch baking dish with non-stick cooking spray.

Heat a large skillet over medium heat. Lay the bacon strips flat in the skillet. It’s okay if the slices overlap slightly, but try not to crowd the pan too much, as the bacon will shrink as it cooks. Cook until slightly crispy, turning occasionally with tongs or a fork. Use tongs to remove it from the skillet and place it on a paper towel-lined plate. Drain all but about 1 tablespoon of the grease from the skillet. Let the bacon cool to room temperature and then chop into bite-size pieces. Set aside.

Heat the skillet with the remaining bacon grease over medium-high heat. Add the onion and cook until tender, stirring occasionally, about 5 minutes. Add the spinach and cook for 2 to 3 minutes or until mostly wilted. Add the garlic and cook for 1 minute. Season with salt and pepper, to taste. Remove from heat and stir in the bacon pieces.

In a medium bowl, whisk together the eggs, milk, Dijon mustard, salt, and pepper.

Spread half of the bread cubes evenly into bottom of the prepared baking dish. Top evenly with half of the spinach bacon mixture. Sprinkle half of the cheese on top.

Repeat this same layering process with bread, spinach bacon mixture, and cheese.

Give the egg mixture a quick whisk and evenly pour over the strata. Sprinkle with parmesan cheese. Gently press down with your hands across to submerge ingredients into the egg mixture.

Cover the strata and refrigerate overnight. When ready to bake, take the dish out of the fridge and let it sit at room temperature for 15 minutes. Preheat the oven to 350 degrees F. Bake the strata uncovered for 50 to 60 minutes, or until golden brown. If the top is getting too brown, you can loosely cover it with foil and continue baking. Remove from oven and let cool 10 minutes before serving.