Dry spinach with a paper or lint-free towel, then finely chop
Cook onion in melted butter in a large skillet over medium heat, stirring frequently, until soft.
Stir in 1/2 teaspoon salt, 1/4 teaspoon pepper, and nutmeg. Cook for 1 minute.
Add spinach, then remove from heat.
Spread one third of bread cubes in a buttered 3-quart casserole dish and top with one third spinach mixture. Sprinkle with one third of each cheese.
Repeat layering twice, ending with cheeses.
Whisk together milk, eggs, mustard and remaining 1/2 teaspoon salt and 1/4 teaspoon pepper in a large mixing bowl and pour evenly over strata.
Chill, covered with plastic wrap, for 8 hours to allow the bread to absorb the custard.
Preheat oven to 350 degrees (F).
Let strata sit at room temperature for 30 minutes before baking.
Bake strata, uncovered, in middle of oven until golden brown and puffed, about 45 to 55 minutes. Let stand 5 minutes before serving.
8 cups of cubed bread is about 12 ounces or 14 - 15 slices of bread.
Dried out, stale bread absorbs the egg mixture more readily. You can cube bread, place on a baking sheet and leave out overnight. Alternately, you can warm in a 180 degree (F) oven for 15 minutes, stirring twice. Allow to completely cool before proceeding with recipe.
A cup of diced ham would be a terrific addition to this recipe!
https://cinnamonsugarandalittlebitofmurder.com