In a fine-mesh sieve, push out as much water as possible from the thawed spinach. Set aside. 

In a large skillet, melt the butter over medium heat and cook onion until it softens, about four to five minutes. Stir occasionally. Add the garlic and cook for 30 seconds. Stir in the drained spinach and crumbled bacon and set aside. 

Whisk the eggs until well combined in a large bowl and add milk, heavy cream, Dijon mustard, salt, pepper and nutmeg. 

Spray a 9-by-13-inch baking dish with cooking spray. Layer a third of the bread cubes on the bottom of the pan and top with a third of spinach mixture. Finish the layer with a third of the cheeses. Repeat the layers two more times in the same order. Pour the egg mixture all over the strata, distributing the mixture evenly. Cover with plastic wrap and refrigerate, preferably eight hours or up to overnight but at least an hour if in a pinch.

Thirty minutes to an hour before baking, remove the strata from the refrigerator to promote even cooking and prevent the pan from cracking in a hot oven. Heat the oven to 325 degrees. Bake the strata uncovered for one hour and 10 minutes. After baking, allow the strata to stand 10-15 minutes to set the custard before serving. Enjoy!