Unless you're on a strict diet, I wouldn't worry about exact measurements. Around breakfast time I'm hungry and I need food in my belly; this is not the time for breaking out the measuring spoons and cups.
Cooking spray or oil
4 egg whites
Small handful of shredded cheese
Handful of fresh spinach leaves, washed and dried
Spray nonstick pan with cooking spray (or use about a teaspoon of cooking oil). Heat over medium-low heat. Add egg whites. When the edges of the egg whites begin to harden and the omelet appears ready to be flipped, add spinach and cheese on one half of the omelet and fold in half.
Reduce heat to low (to allow omelet to continue cooking but prevent it from burning) and cook until egg whites are no longer runny, cheese is melted, and spinach is wilted. Season with salt and pepper to taste or enjoy your omelet with some flavorful salsa.
Have a beautiful day!