Begin by gathering all your ingredients. Dice the onion and red bell pepper, and chop the baby spinach. If your bread is not already cubed, cut it into bite-sized pieces.
In a skillet, heat the olive oil over medium heat. Add the diced onion and red bell pepper. Sauté for about 5 minutes, or until they are soft and fragrant. Stir in the chopped spinach and cook until it wilts. Remove from heat and let it cool slightly.
In a large mixing bowl, combine the cubed bread, cheddar cheese, and Gruyère cheese. Mix them well to ensure the bread is evenly coated with cheese.
In another bowl, whisk together the eggs, whole milk, salt, black pepper, and dry mustard until well combined. This creamy mixture is what will soak into the bread.
Add the sautéed vegetables to the bread and cheese mixture. Pour the egg mixture over the top and gently fold everything together until the bread is well coated. Be careful not to mash the bread cubes.
Transfer the mixture to a greased 9x13 inch baking dish, spreading it evenly. Cover the dish with plastic wrap or aluminum foil and let it rest in the refrigerator for at least 30 minutes to overnight. This allows the bread to soak up the egg mixture.
When you’re ready to bake, preheat your oven to 350°F (175°C). Remove the plastic wrap and bake the strata for 45-55 minutes, or until the top is golden brown and a knife inserted in the center comes out clean.
Once baked, let the strata cool for a few minutes. Garnish with fresh chopped parsley before serving. Enjoy it warm, perhaps with a side of seasonal fruit or a light salad.
