Heat a large non-stick skillet over a medium heat and cook the onions, garlic and oregano in the cooking fat until the onions are browned and soft, about 25 minutes. Season to taste with sea salt and freshly ground black pepper.
2 tablespoon cooking fat, 4 large onions, 1 garlic clove, 2 teaspoon oregano, Sea salt and freshly ground black pepper to taste
Add the balsamic vinegar and water and continue cooking to reduce the liquid so that the onions are well coated in a thick liquid.
2 tablespoon balsamic vinegar, 2 tablespoon water
In a bowl, whisk the eggs, add the minced parsley and season to taste with sea salt and freshly ground black pepper.
10 eggs, 2 tablespoon parsley, Sea salt and freshly ground black pepper to taste
Pour the egg mixture into the skillet with the onions and combine well.
Still over a medium heat, gently stir and shake the pan at the same time until small curdles start to form. At this point, let the frittata cook without stirring so it can hold its shape.
When the frittata seems set, loosen it from the bottom of the skillet, place a large plate over the skillet and flip the frittata into the plate. Return it to the skillet, uncooked side down.
Cook for a further 2 minutes to set the bottom and transfer to a large plate.
Cut the frittata in wedges and serve hot or at room temperature, topped with extra balsamic vinegar if desired.