Spray a 9x13 baking dish with oil.

Heat up your pan to medium-high heat and brown the sausage just a little bit. Do that even if the sausage is already fully cooked.

Cover the baking dish with the sausage.

Add olive oil to the same pan, add in the onion, garlic, spinach, and bell peppers. Cook until the spinach is wilted and vegetables have become tender. It will take around 6-7 minutes in total. Season with salt and black pepper to taste.

Cover the sausage inside the baking dish with the cooked vegetables.

In a large bowl, crack in the eggs, add in milk, ½ cup of grated cheddar, all of the spices, and salt with black pepper. Whisk all of those ingredients together.

Pour the egg mixture over the vegetables inside the baking dish. Top up with the remaining ¼ cup of grated cheddar cheese.

Cover the baking dish with plastic wrap and keep it in the fridge for about 2 hours to let it set.

Preheat the oven to 375F.

Bake the casserole for 45 minutes until the edges are golden brown. Let it cool down before serving.

Store in the fridge and reheat in the microwave before eating.