Step 1: Preheat the Oven - Start by preheating your oven to 350°F (175°C). This will ensure that your strata cooks evenly and thoroughly.
Step 2: Sauté the Vegetables - In a skillet, heat the olive oil over medium heat. Add the diced onion and red bell pepper, cooking until softened, about 5-7 minutes. Stir in the chopped spinach and cook for an additional 2 minutes until wilted. Remove from heat and set aside.
Step 3: Prepare the Bread - In a large mixing bowl, combine the cubed day-old bread with the sautéed vegetables. Toss to mix evenly, ensuring the bread absorbs the flavors.
Step 4: Add the Cheeses - Sprinkle the shredded cheddar and Gruyère cheeses over the bread and vegetable mixture. Gently fold the ingredients together, allowing the cheese to distribute throughout the strata.
Step 5: Whisk the Eggs and Milk - In another bowl, whisk together the eggs, whole milk, salt, black pepper, and dry mustard until well combined. This mixture will bind all the ingredients together.
Step 6: Combine Everything - Pour the egg mixture over the bread and vegetable mixture. Gently stir to ensure that all the bread is coated with the egg mixture. Let it sit for about 10-15 minutes to allow the bread to soak up the egg.
Step 7: Bake the Strata - Pour the mixture into a greased 9x13 inch baking dish, spreading it out evenly. Bake in the preheated oven for 45-50 minutes or until the strata is puffed and golden brown on top.
Step 8: Garnish and Serve - Once baked, remove the strata from the oven and let it cool for a few minutes. Garnish with freshly chopped parsley before slicing and serving.
