Preheat your oven to 350°F (175°C) and lightly grease a baking dish with butter or oil

Heat a large skillet over medium heat and add olive oil or butter

Add chopped onions and sauté for 2–3 minutes until slightly softened

Stir in minced garlic and cook for another 30 seconds until fragrant

Add the breakfast sausage and cook while breaking it apart with a spatula until fully browned and no longer pink

Drain excess grease from the skillet to avoid a greasy casserole

Add diced bell peppers to the sausage mixture and cook for 2–3 minutes until slightly tender

Remove the skillet from heat and allow the mixture to cool slightly

In a large mixing bowl, crack the eggs and whisk until smooth and slightly frothy

Pour in heavy cream and whisk again until well combined

Add salt, black pepper, and paprika to the egg mixture and mix thoroughly

Stir in half of the shredded cheddar cheese and mozzarella for a rich base

Spread the cooked sausage and vegetable mixture evenly across the bottom of the greased baking dish

Pour the egg mixture evenly over the sausage layer, ensuring it spreads into all corners

Sprinkle the remaining cheese evenly on top for a golden crust

Place the dish in the oven and bake for 35–40 minutes until the center is set and the top is lightly golden

Check doneness by inserting a knife in the center; it should come out clean

Remove from oven and let the casserole rest for 10 minutes before slicing

Garnish with fresh parsley if desired

Slice into squares and serve warm