In a large skillet over medium heat, heat 1 tablespoon of oil. Add the mushrooms and sauté them for 4 minutes or until they start to brown. Add a small pinch of salt along withe onions and garlic and let cook for 2 minutes. Remove to a plate. Add the second tablespoon of oil, if needed, and sauté the peppers for 1 minute. Add in the baby spinach and allow it to wilt, remove from heat, set aside.
Generously spray a 9×13 baking dish with nonstick cooking spray. Lay the shredded potatoes in the bottom of the dish and press to make sure they are spread out evenly. Add the veggies (both the mushrooms and the pepper mixtures) over the potatoes, set aside.
Position a rack in the center of the oven and preheat the oven to 375ºF if you are baking the casserole immediately.
In a large bowl, whisk together the egg, half and half, hot sauce, and a big pinch of salt and pepper. Pour the egg mixture over the prepared veggies. Sprinkle the cheese over the top and another small pinch of salt and pepper.
Bake the casserole, uncovered, until the cheese melts, and the top gets in and golden, 45-50 minutes. Allow the casserole to cool for 10 minutes before slicing and serving.
TO MAKE AHEAD – prepare the casserole as directed, cover with plastic wrap and refrigerate overnight or as required (not to exceed 24 hours) and allow it to come to room temperature for 20 minutes before baking.
The half and half can be replaced with any nondairy milk such as almond, soy, cashew, etc.
OTHER ADD INS – I kept this recipe vegetarian, but you can certainly add in browned sausage, ground turkey/beef, shredded chicken or ham. The cheeses I recommend for this casserole are cheddar (mild or sharp), mozzarella, and pepper jack, or swiss).