Fontina Speck and Onion Strata

http://www.marthastewart.com/345558/fontina-speck-and-onion-strata?center=276958&gallery=275554&slide=285826

This strata -- rich, cheesy, and studded with onion and salty speck -- is refrigerated for several hours to ensure that it soaks up all the liquid.
Martha Stewart Living, November 2010

Yield Serves 8
Ingredients

3 tablespoons unsalted butter
1 large sweet onion, preferably Vidalia (about 1 1/2 pounds), thinly sliced
Coarse salt and freshly ground pepper
10 large eggs, lightly beaten
3 cups whole milk
1 cup grated Parmigiano-Reggiano cheese
2 garlic cloves, minced
2 teaspoons chopped fresh thyme
10 cups day-old rustic bread (from 13 ounces), cut into 3/4 -inch cubes
4 ounces sliced speck or ham, coarsely chopped (1 1/4 cups)
3 cups shredded fontina (8 ounces)
Vegetable oil cooking spray

Directions

Heat butter in a large high-sided skillet over high heat. Cook onion until lightly caramelized, 10 to 12 minutes (adjust heat if browning too quickly). Season with 1/2 teaspoon salt. Transfer onion to a plate, and let cool.
Meanwhile, whisk together eggs, milk, Parmigiano-Reggiano, garlic, thyme, and 1 teaspoon salt in a bowl. Season with pepper. Add bread cubes, and toss well to combine. Toss in onion and speck. Refrigerate, covered, for at least 2 hours.
Preheat oven to 325 degrees. Spread half the bread mixture on the bottom of a 10-by-14-inch oval baking dish. Sprinkle with 2 cups fontina. Top with remaining bread mixture. Coat a large piece of parchment with cooking spray. Cover strata with parchment, then with foil. Bake for 1 hour. Uncover, and sprinkle with remaining cup fontina. Raise oven temperature to 375 degrees. Bake until top is golden brown and crusty, 20 minutes more. Let rest for 15 minutes. Serve warm.