Preheat oven to 375.
Beat together the eggs with 1/2 tsp salt and a few grinds of pepper, Stir in the cheese and chopped spinach and set aside.
Coat a large oven-proof, non-stick skillet with cooking spray. Add the onions, bell pepper, and mushrooms and cook over medium heat until they are slightly browned, about 5 minutes. Add frozen potatoes, sprinkle with remaining 1/2 tsp salt, and stir to cook with the onion for a minutes or two, just until the frost melts off of the potatoes.
Remove veggies from heat, let cool for a few minutes, then add to the eggs and spinach. Whip skillet clean and set aside. Mix veggies vigorously as you add them into the eggs so the mixture doesn't scramble too much. (I'll be honest. I've been impatient and just added the screaming-hot veggies to the eggs; it was just fine.)
Return skillet to medium heat and coat with cooking spray again. Add in egg and veggie mixture. Cook for just about a minute or two, just to make sure the bottom gets brown. Add skillet into oven and bake for about 10 minutes, until the eggs are set in the middle. You can cook for a little bit less or more, depending on how you like your eggs. I like mine a teensy bit underdone, so I took mine out at about the 9 minute mark.
Let cool for a few minutes before serving, Cut into 6 equal portions.