1 (10-ounce) package frozen spinach, thawed
1 1/2 cups finely chopped yellow onion
1 cup finely sliced fresh mushrooms
3 tablespoons extra-virgin olive oil
1 pound sweet Italian sausage (or another variety of your choosing)
1 teaspoon salt, divided
1/2 teaspoon black pepper, divided
1/4 teaspoon nutmeg
1/2 cup drained and chopped, marinated artichoke hearts (optional)
1/2 cup roasted, peeled and diced red bell peppers (optional)
1/2 cup sliced black olives (optional)
1 pound loaf French or Italian bread, cut into 1-inch cubes to measure 8 cups
2 cups shredded Gruyere (about 6 ounces)
1 cup shredded Parmesan
9 large eggs
2 3/4 cups milk
2 tablespoons Dijon mustard (or whole-grain mustard for a heartier flavor)

Place the thawed spinach in a strainer and press firmly to remove all moisture, then finely chop with a chef's knife and set aside.

Saute the onions and mushrooms in the olive oil until mushrooms are softened and translucent and mushrooms turn slightly golden around edges. Add the sausage and brown thoroughly, breaking it into small chunks as it cooks.

Stir in 1/2 teaspoon of the salt, 1/4 teaspoon of the pepper and the nutmeg; continue to saute for 1 more minute. Add greens and saute for another minute as you distribute them throughout mixture. Remove from heat and add the artichoke hearts, bell peppers or olives, if desired.

Grease a 3-quart, ceramic baking dish. Spread 1/3 of the bread cubes in bottom of dish, top with about 1/3 of sausage mixture, then 1/3 of both of the cheeses. Repeat layering 2 more times, ending with cheeses.

In a large bowl, whisk together the eggs, milk, mustard and remaining salt and pepper. Pour evenly over casserole and cover with foil or plastic wrap. Chill overnight (or for at least 5 hours) so bread can soak up egg mixture.

Bake casserole, uncovered, in a 350-degree oven for 45 to 55 minutes, until puffed, golden-brown and cooked in center (when a kitchen knife inserted comes out clean). Let stand for about 5 minutes before serving.

Makes 6 to 8 servings.