01 -
Preheat oven to 375°F. Lightly grease a 9x13-inch casserole dish with butter or nonstick spray.
02 -
In a large mixing bowl, whisk eggs, whole milk, Dijon mustard, garlic powder, smoked paprika, salt, and black pepper until fully combined.
03 -
Arrange 6 slices of sourdough bread in the base of the casserole dish, trimming to fit as needed. Evenly spread half the softened cream cheese over the bread slices.
04 -
Distribute half of the sharp cheddar, Gruyère, and mozzarella cheeses evenly over the bread. Add half of the sliced tomatoes, red onion, spinach leaves, and chopped bacon if using.
05 -
Repeat with remaining bread slices, cream cheese, shredded cheeses, tomatoes, red onion, spinach, and bacon to create a second layer.
06 -
Pour the egg and milk mixture evenly over the entire casserole. Gently press down on the layers to help the bread absorb the liquid.
07 -
Drizzle melted unsalted butter over the top layer. Sprinkle with chives or green onions.
08 -
Cover casserole with foil and bake for 25 minutes.
09 -
Remove foil and bake an additional 10 minutes, until golden brown and bubbling.
10 -
Allow casserole to rest for 5 to 10 minutes before slicing into squares. Serve warm.