First, prepare the filling. A basic rule of thumb is that you need 1/4-1/3 cup of filling for every two eggs. Dice any vegetables you are using. 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. You can also just leave them raw. If you are making a cheese omelette, either slice the cheese thinly or grate it finely and put aside.
Crack the eggs into a small mixing bowl. Stir gently with a fork until well-beaten. Add 1 tablespoon milk (or water), salt and pepper, and any herbs, and set aside.
Heat a 6- to 8-inch omelette pan over high heat until very hot (approximately 30 seconds). Add 1 teaspoon butter, making sure it coats the bottom of the pan. As soon as the butter stops bubbling and sizzling (and before it starts to brown), slowly pour in the egg mixture.
Tilt the pan to spread the egg mixture evenly. Let eggs firm up a little, and after about ten seconds shake the pan a bit and use a spatula to gently direct the mixture away from the sides and into the middle. Allow the remaining liquid to then flow into the space left at the sides of the pan.
Continue to cook for another minute or so until the egg mixture holds together. While the middle is still a little runny, add the filling. Put in sautéed vegetables or fruit first, near the center, then sprinkle any cheese on top.
Tilt the pan to one side and use the spatula to fold approximately one third of the omelette over the middle. Shake the pan gently to slide the omelette to the edge of the pan.
Holding the pan above the serving plate, tip it so the omelette rolls off, folding itself onto the plate. The two edges will be tucked underneath.
Serve with toast and orange wedges.