Preheat oven to 375°F (190°C) and grease a 9x13 inch baking dish.
In a large skillet over medium heat, cook the Italian sausage until browned, breaking it into crumbles; drain excess fat.
In the same skillet, sauté the diced onion and bell pepper for 5 minutes until softened, adding the garlic in the last minute.
In a large mixing bowl, whisk together the eggs, milk, oregano, basil, salt, and black pepper until well combined.
Spread the cooked sausage and sautéed vegetables evenly across the bottom of the prepared baking dish.
Sprinkle 3/4 cup of mozzarella cheese over the sausage and vegetable layer.
Pour the egg mixture over the ingredients in the baking dish.
Top with the remaining mozzarella and the grated parmesan cheese.
Bake for 35 to 45 minutes, or until the eggs are set and the top is golden brown.
Allow the casserole to rest for 10 minutes before slicing and serving.