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 9x13 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.
In a large bowl, whisk together the egg, milk, hot sauce, and salt and pepper. Pour the egg mixture over the prepared veggies.
Sprinkle with the cheese and anther 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.