Melt butter in a large skillet. Add onion and garlic and saute until soft and translucent for approximately 5 minutes. Add spinach and stir until cooked and wilted, approximately 2 minutes. Remove from heat and set aside.
Meanwhile, cook the bacon in another saute pan until browned and fat rendered.
In a large bowl, add all of the eggs, milk, cream, salt, pepper, and ground mustard. Whisk until well combined.
Grease your baking tray with butter and add cubed bread in a single layer. Add spinach and onions, spreading evenly as possible over the bread. Crumble bacon and sprinkle over the top of the spinach and onion mixture. Sprinkle feta and Parmesan cheese in an even layer over the top. Then gently pour in the egg and milk mixture.
Cover with plastic wrap and store in fridge for at least an hour and up to 24 hours.
Pre-heat oven to 375° F. Remove baking dish from the fridge and let come up to room temp for 15-20 minutes.
Baked covered lightly with aluminon foil for 30 minutes. Remove foil and bake for another 25 minutes uncovered.
Enjoy!
Notes
Prepare in AdvanceOne of the best things about this recipe is you can prep it all in advance and just toss it in the fridge. Make it in the am for dinner, or make it the night before for breakfast. SubstitutionsBread - Use any of your favorite thick-sliced white bread. Spinach - You can use frozen by thawing the spinach and squeezing out the excess liquid before adding it to your saute pan with the onion, garlic, and butter. Mustard - You can use a tablespoon of mustard in place of ground. Yellow for a mild flavor, or dijon for a kick!Cheese - Use your favorites. Mozzarella and cheddar are also delicious. But we love the classic combo of spinach and feta. Milk / Half and Half - Use whatever combination of the two you have. The more half and half, the richer the flavor. You could also just use all whole.