Preheat oven to 350° F. Grease an 8x8- or 9x9-inch casserole dish well with butter (it works better than cooking spray!). (See Cooking Notes about different pan materials.)
In a medium-sized skillet, heat 1 teaspoon oil and cook the turkey sausage over medium-high until no longer pink, breaking it up with a wooden spoon as it cooks. Drain off any grease. Sprinkle the crumbled meat in the bottom of the casserole dish.
Add another teaspoon of oil, if needed, and the red bell pepper to the pan and saute just until soft, about 4-5 minutes, seasoning with a pinch of red pepper flakes and lightly with salt and black pepper. Add the garlic at the end of the cook time, stirring around for about 30-60 seconds. Be careful not to burn the garlic. Remove from the heat.
Spread the red bell pepper mixture over the turkey sausage. Sprinkle the spinach over the peppers. Finally, top with 1/2 cup mozzarella cheese and then all the feta cheese.
In a medium mixing bowl, whisk together the eggs, Parmesan, milk, and 1/2 teaspoon ground black pepper. Pour egg mixture over the top of the casserole. Sprinkle the remaining 1/4 cup mozzarella cheese over the top last of all.
Bake for 40-50 minutes (keep a close eye on it!), rotating the pan around once. Casserole is done when the top looks puffy and is slightly golden brown and the center just barely jiggles when you give a gentle shake of the pan. You can also insert a knife in the middle and if it comes out clean, it’s done.
Let cool slightly. Then slice and serve warm. Leftovers can be stored in the fridge and used for breakfast and lunches for up to 5 days.