Preheat the Oven: Set your oven to 375°F (190°C).
Prepare the Baking Dish: Grease a 9x13-inch baking dish with cooking spray or butter.
Sauté Vegetables: In a skillet, sauté the diced bell pepper and onion over medium heat until softened, about 5 minutes.
Mix Wet Ingredients: In a large bowl, whisk together the eggs, cottage cheese, and shredded cheddar cheese until well combined.
Add Vegetables and Seasoning: Stir in the sautéed vegetables, chopped spinach, salt, and pepper.
Transfer to Baking Dish: Pour the mixture into the prepared baking dish, spreading it evenly.
Bake: Place in the preheated oven and bake for 35–45 minutes, or until the top is golden brown and a knife inserted into the center comes out clean.
Cool and Serve: Allow the casserole to cool for 10 minutes before slicing and serving.