Spinach and Cheese Strata

INGREDIENTS

3 tbsp

unsalted butter

1 1/2 cups

onion, finely chopped

2 10 ounce packages

frozen chopped spinach, thawed and drained

1 tsp

salt, divided

1/2 tsp

pepper, divided

freshly grated nutmeg

8 cups

cubed French or Italian bread, cut into 1-inch cubes

6 oz

coarsely grated Gruyere (about 2 cups)

2 oz

finely grated Parmesan (about 2/3 cup)

large eggs

2 3/4 cups

milk