1. In a saucepan, boil water and add in the potatoes; cook until done.
2. In a shallow pan, heat olive oil and stir in the onion. Sauté for 10 minutes.
3. When onion is about to be done add in the garlic and sauté again for a few seconds.
4. Take off the pan from the heat and let it cool.
5. In a large bowl, add in the egg, spinach and half of the cheese.
6. Add in the onion mixture to the egg and stir well with a wooden spoon.
7. Drain the potatoes and thinly slice them, add into the egg mixture.
8. Heat a large sauté pan and add in the olive oil.
9. Pour the mixture into the pan and top with remaining cheese and tomatoes. Cook until the edges are firm.
10. Place the pan into the grill to cook for another 5 – 6 minutes on medium heat.
11. Slice the frittata and serve along with fresh fruit juice and toast.