Place the defrosted spinach into a piece of cheesecloth or a clean kitchen towel and squeeze out all the excess liquid over the sink. Set aside.
In a large skillet, add 3 tablespoons unsalted butter and melt over medium heat. Once melted, add the minced onion, along with a pinch of salt and pepper to season, and sauté on medium heat until soft, about 4-5 minutes, stirring often. Once the onion has softened, remove the skillet from the heat, stir in the drained spinach and diced peppers and set aside.
In a large mixing bowl, whisk together 9 large eggs, 2 cups milk, ¾ cup heavy cream, 2 tablespoons Dijon mustard, ¼ teaspoon grated nutmeg, and ½ teaspoon salt, and ¼ teaspoon pepper together until well combined.
Spray a 9-by-13-inch baking dish or 3-quart casserole dish with cooking spray or brush with melted butter. Layer a third of the bread cubes on the bottom of the pan and top with a third of the spinach mixture. Sprinkle with ⅓ of the Gruyere cheese and ⅓ of the parmesan cheese over the spinach mixture. Repeat the layers two more times in the same order.
Pour the egg mixture all over the strata, distributing the mixture evenly. Use your hands to gently push the bread into the egg mixture if needed. Cover with plastic wrap and refrigerate for 8-24 hours.
When ready to bake, remove the casserole from the refrigerator and let it sit at room temperature for 30 minutes. Preheat the oven to 350℉ (175℃).
Remove plastic wrap from casserole, and bake uncovered, for 20 minutes. Loosely cover with aluminum foil and bake for an additional 30-35 minutes or until browned on the sides and puffy in the center.
Let cool slightly and then slice and serve the strata.
Video
Notes
Storage: Store leftover baked and cooled strata, in an airtight container in the refrigerator for 3 days. Reheat individual slices in the microwave in 30-second intervals on a heat-safe plate or reheat the entire casserole covered with foil in a 350℉ for 20-30 minutes or until warmed through. Milk/Cream: Feel free to use half in half to replace both the milk and cream. Alternatively, omit the heavy cream and replace it with milk.