Preheat oven to 375 degrees
While the oven is pre-heating, prepare your vegetables. I pre-cooked the white potatoes and the sweet potatoes to speed the cooking time. Slice the potatoes in half length wise, then cut into thin slices. Slice the onion, green pepper, cherry tomatoes and mushrooms. If you purchase the black olives whole, make sure to slice them. Set cut vegetables aside.
Crack 6 eggs and 6 egg whites into a bowl then add the salt and pepper. Whisk until combined.
Over medium heat, heat oil in a cast-iron skillet. Add onions and green pepper and stir until browned. Stir in the sliced mushroom and olives and let brown about 1 minute. Then, add the white potato and sweet potato and gently toss, trying not to mash the potatoes. Once combined, add the spinach. Stir everything gently and use a spayula to make sure all ingredients are evenly distributed in the skillet.
Pour the egg mixture over the vegetables, and allow it to cook for 1-2 minutes, or until the egg begins to set.
Transfer the pan to a preheated oven (375 degree). Let it cook in the oven for ~15 minutes, or until the eggs are set but not brown. If after 15 minutes the eggs are still bubbling on the top, allow another minute to cook. (Don't walk away from the oven at this point.) Once eggs are set remove from the oven.
Allow to sit a few minutes. Slice frittata in the cast-iron skillet or slide it out of the skillet onto a cutting board. Slice into wedges. The nutrition information provided is for 4 servings. If you want smaller portions, slice it into 8 wedges.