Crack the eggs into a bowl and beat to blend using a fork or whisk. Incorporate as much air into them as you can for the lightest, fluffiest texture—if you have an immersion blender use it for an ultra-fluffy omelet.
To prevent shell in the eggs, crack them firmly on a hard, flat surface rather than the rim of a bowl.
Coat a nonstick skillet with cooking spray. Heat the skillet over medium—avoid cooking the eggs over high heat as it can make them tough. Add ½ tsp. butter per egg to the skillet. Pour the beaten eggs into the skillet when the butter melts and is foamy, but not browned. Cook the eggs just until the edges begin to set, about 1 min.
Pull the eggs away from the edge of the pan using a flexible heat-proof spatula. Swirl the pan to let the liquid egg to flow into the space you created. Continue pulling the edges into the center until the egg is just set. It should look a little runny on top.
To prevent overcooking the omelet, remove the pan from the heat once the eggs are set. The eggs will finish cooking from the heat in the skillet.
Add cheese or fillings to one half of the omelet. Fold the other half over the fillings using the spatula. Let the omelet stand for 2 min. to melt the cheese and warm the fillings. Slide the omelet from the pan onto a serving plate using the spatula.