Heat a large non stick frying pan over medium low heat. Add the onions and cook, stirring occasionally for 25 minutes until deep golden brown. Stir through the balsamic, sugar, salt and pepper along with the oregano
Whisk together the eggs, cream, salt and pepper and pour into the pan. Move the mix around gently with a spatula to cover the base of the pan underneath the onions
Cook the frittata over low heat for 20-25 minutes then once the top has just set scatter over the feta. Continue cooking on the stove top for another 5 minutes then transfer to a pre heated oven grill. Cook for 5 minutes or until golden and set
To serve, cut frittata into wedges and serve with a simple rocket salad