1
In a large skillet over medium heat, heat 1 tablespoon of oil. Add the mushrooms and saute 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 saute the peppers for 1 minute. Add in the baby spinach and allow it to wilt, remove from heat, set aside.
2
Generously spray a baking dish with nonstick cooking spray. Add the veggies (both the mushrooms and the pepper mixtures), set aside.
3
Position a rack in the center of the oven and preheat the oven to 375 if you are baking the casserole immediately.
4
In a large bowl, whisk together the egg, skim milk 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.
5
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.