Spray a 7 x 11" or 9 x 13" casserole dish with non-stick spray. Set aside. If you are going to bake this casserole immediately, preheat the oven to 375ºF (191ªC).
In a large skillet over medium heat, brown the sausage. If you are using fully cooked sausage, you may skip this step. I like to brown my sausage just a little bit, even if it is already fully cooked. Spoon the browned sausage evenly into prepared casserole dish. Set aside.
Add a little olive oil to the same large skillet (I often skip this since there is some grease left from the sausage). Sauté the garlic, spinach, peppers, and onion until vegetables are tender and spinach is wilted, about 6-7 minutes. Season with salt and pepper during the last minute or so.
Remove from heat and add the cooked vegetable mixture to the sausage in the prepared casserole dish and roughly stir together to distribute mixture evenly. Set aside.
In a medium size bowl, whisk together the eggs, milk, and ¼ cup of the shredded cheese. Pour egg mixture evenly over sausage and vegetable mixture. Sprinkle with remaining ¼ cup of shredded cheese and a little more salt and pepper. You can either bake breakfast casserole immediately or cover the dish with plastic wrap or foil and refrigerate for up to 24 hours to bake later.
When you are ready to bake the casserole: if you haven't already, preheat the oven to 375ºF (191ºC). Then, bake casserole for approximately 45 minutes or until edges start to brown. Allow to cool at least 15 minutes before serving. Store leftovers in the refrigerator up to 5 days. Reheat individual servings in the microwave as needed. Baked casserole may be frozen, up to 2 months. Thaw in refrigerator overnight and bake at 350ºF (177ºC) for about 20 minutes or until heated through.
To freeze this casserole: you can freeze unbaked casserole up to 2 months. Cover very tightly with plastic wrap and foil. Thaw overnight in the refrigerator and continue with step 6.
Sausage: I typically use fully cooked sausage. You are welcome to use whatever kind of sausage you like. Consider using ground beef, ground turkey, chicken, or any leftover meat you need to use up. Vegetarians:feel free to leave out the meat completely. Consider adding an extra vegetable in its place.
Olive oil: if you are browning meat, you can just use the oil leftover in the pan for the vegetables.