First, prepare the filling. A basic rule of thumb is that you need one quarter to one third cup of filling for every two eggs. If you are using a filling that needs to be cooked — such as apples, mushrooms, onions, peppers, leeks — quickly sauté in a small frying pan with 1 teaspoon of the butter. If you are making a cheese omelette, either slice the cheese thinly or grate it finely and put aside.