Preheat oven to 400 F and arrange a rack in the middle of the oven.
If using potatoes, bring a large pot of salted water to a boil. Place the potatoes in the water, and let them cook about 8 minutes, or until they can be pierced with a fork. Drain and set aside.
If you have a large oven-safe skillet, like a cast iron, use that. If not, use a large skillet. Heat to medium heat and add 1 tablespoon of olive oil
Start with the vegetables that take the longest to cook. Saute vegetables in the pan, adding more vegetables as you go. Add another 1/2 of teaspoon salt and cook until all vegetables are soft inside and crispy on the outside. Add any "flavor additions" and stir.
Spread the vegetables in an even later, flattening with a spatula. Sprinkle cheese on top and let it start to melt. (If you don't have an oven-safe skillet, transfer the vegetables to a large baking dish, and then add the cheese and eggs. You may need to cook the dish another 5-10 minutes in the oven)
Whisk eggs, milk, and 1/2 teaspoon salt together in a bowl.
Pour the egg mixture into the skillet. Tilt the pan so the eggs settle evenly over all the vegetables. Cook for a minute or two until you see the eggs at the edges of the pan beginning to set.
Transfer to the oven and bake for 8 - 10 minutes, or until the egg are set. You can check this by poking a knife into the center of the pan. If raw egg is left on the knife, then cook for another 5 minutes and check.
Cook for about 5 minutes and serve! Keep leftovers in the fridge for 4 days or wrap tightly in plastic wrap, and store for up to 6 months in the freezer