Strata is essentially a savory bread pudding, and lends itself beautifully to variations like spinach, mushrooms, or peppers.
2 tablespoons unsalted butter
1 large Spanish onion, minced
1 teaspoon kosher salt, divided
2 cups frozen spinach thawed or fresh, checked spinach, cut in ribbons
8 cups challah, cut into 1 inch cubes
8 ounces cheddar cheese, shredded
2 ounces grated Parmesan cheese
2 3/4 cups whole milk
2 tablespoons Dijon or yellow mustard
1/2 teaspoon fresh ground black pepper
1/4 teaspoon fresh grated nutmeg
Melt the butter in a large skillet over medium heat and then add the onion, cooking until it’s soft and translucent. Stir in the spinach and 1/2 teaspoon salt, and cook for three minutes more. Remove from heat and set aside.
Butter a 9 x 13 x 3 inch casserole dish. Layer the bread cubes, cheeses, and cooked spinach mixture, making three layers.
Whisk the eggs and milk together, and then add the mustard, salt, pepper, and nutmeg. Pour over the strata. Cover with plastic wrap and chill, for several hours or overnight.
Set the dish out to come to room temperature. Preheat oven to 350 degrees, and bake for 45 – 50 minutes, or until the edges have browned slightly and pulled away from the edges.