1. Sauté all the vegetables, except the spinach in a non-stick pan (cast iron is the best) for about 5-10 minutes or until the carrot and potato are soft.
2. Add the spinach in the last 1 minute.
3. Mix the eggs in a bowl with a whisk or a fork and pour them in the pan with the vegetables.
4. Season the frittata to taste with salt and pepper.
5. Push the vegetables with a wooden spatula to spread the egg mixture evenly and cook at medium heat until the bottom is set and lightly brown.
6. At that point you have to finish cooking the frittata in a preheated oven on "broil".
You have to be careful to choose a pan with oven-proof handle, so you can put the pan in the oven.
7. Spread the cheese over the frittata and broil it for about 5-10 minutes or until the top is set and the cheese is nicely melted.
Tip: If you don't have a frying pan with oven-proof handle, you can cook the frittata entirely on the stove top, however you would have to flip it in order to cook the other side. You can do that safely by flipping the frittata on a plate first and then sliding it back in the frying pan.