Blend the onions in a food processor then heat them in a pan with 2 tbsp of olive oil. Cook until the onions are transparent (about 10 minutes). Then remove from the heat and let them cool. Beat the eggs in a bowl with salt and pepper. Then add the the onions and stir. Heat the pan where you cooked the onions (still oiled) and, when very hot,  pour in the mixture. Cook the frittata on a low heat on one side (about 5 minutes) then take a large plate, place it upside down on the the pan and holding it firmly turn the pan and the frittata over into it. Slide the frittata back into the pan and cook the other side. Remove from the heat and serve hot or at room temperature.