Cook bacon, crumble and set aside. Add onion and garlic to pan and cook. Stir in bacon and spinach.
Whisk the eggs until well combined in a large mixing bowl and add milk, heavy cream, Dijon mustard, salt, pepper and nutmeg.
Layer the bread with the bacon mixture and Gruyere and Parmesan. Pour the egg mixture all over. Cover and refrigerate for at least an hour but preferably overnight. Bake.
After one taste, this recipe will be a new brunch tradition! It's great for feeding a crowd, and you'll love how the make-ahead preparation makes hosting a breeze. Enjoy!