In bowl, whisk or fork-beat eggs, milk or cream (if using) and salt. Melt butter in 7- or 8-in. (18- or 20-cm) skillet on medium heat; swirl to coat bottom.
Beat egg mixture again; pour into skillet. The egg mixture should just begin to set around the edges (reduce heat if it sets too fast). Working quickly, use the tip of the spatula to push 3 or 4 "furrows" into the centre of the omelette, allowing the liquid egg mixture to fill gaps. The furrows make the omelette fluffier.
While omelette is still moist, sprinkle on fillings (if using). Use the spatula to fold the omelette in half. To allow omelette to puff up, cover skillet with lid for about 30 sec. Serve immediately.
For less browning, use 1 tsp (5 mL) butter and 1 tsp (5 mL) vegetable oil.