Instructions:

1. Peel the onion and slice it thinly.
   
2. Heat a little olive oil in a pan over low heat. Add the onion and sauté, stirring regularly, until soft and golden (about 20 minutes).
   
3. Transfer the onion to a bowl and let it cool slightly.
   
4. In a large bowl, lightly beat the eggs with a fork. Add the cooled onion, grated cheese (if using), salt, and black pepper. Mix well.
   
5. Heat a little more olive oil in a small pan and pour in the egg mixture. Cover with a lid and cook over low heat, occasionally shaking the pan to prevent sticking.
   
6. Once the top is no longer runny (after about 15 minutes), carefully flip the frittata and cook for another 5 minutes on the other side.