Adapted from Chicken and Egg
2 tablespoons unsalted butter
1 medium onion, sliced
5 cups lightly packed cubed (3/4- to 1-inch cubes) challah or another egg bread
2 cups lightly packed shredded butternut squash (from about 3/4 pound squash)
One 9- to 10-ounce package frozen chopped spinach, thawed and squeezed dry
3 1/2 cups half-and-half
1 teaspoon kosher salt
1/4 teaspoon freshly ground pepper
2 cups lightly packed shredded aged Gouda cheese (6 ounces)
1. Lightly butter a 13 x 9-inch glass baking dish or coat with nonstick spray. Melt the butter in a medium skillet over medium heat. Add the onion and cook for 8 to 10 minutes or until the onion is golden brown, stirring occasionally.
2. Meanwhile, toss the challah and squash together in a large bowl and arrange in the baking dish. Scatter the browned onion and the spinach over the bread mixture. Whisk the eggs in a large bowl until blended and smooth. Whisk in the half-and-half, salt, and pepper until combined. Pour the egg mixture over the bread and vegetables. Make sure all of the bread mixture is moistened, pressing on the bread cubes if necessary to coat with the egg mixture. Sprinkle the cheese over the top. Cover with plastic wrap and refrigerate overnight.
3. When ready to bake, preheat the oven to 350°F. Remove the plastic and let the strata sit at room temperature while heating the oven. Bake for 50 to 60 minutes or until the strata is puffed, the top is golden brown, the center is set, and a knife inserted in the center comes out moist but with no milky reside. Let sit for 5 minutes before cutting.