This is one of my favorite omelettes to make - mushroom, feta and spinach. If you have some kalamata olives those are great too, and tzatziki is a great garnish.
Beat 4 eggs until fully blended, add some freshly ground black pepper and a little fresh or dried tarragon. Get out a bag of frozen spinach so it can thaw a bit.
Slice the mushrooms - portobellos or creminis are really good here, but white mushrooms are good too. Saute the mushrooms in a little olive oil until cooked and put into a bowl.
Pour the eggs into the pan (same one you used for mushrooms) and keep the pan temperature to medium - you don't want to burn the eggs! Sprinkle a little spinach around one half of the pan, on top of the eggs, and them add the mushrooms and a little feta on top (also the olives if you have them, sliced in halves or quarters).
Once the egg all apears cooked (firm, not dry, but not runny) use your spatula to gently release the edge of the egg around the edge of the pan. Get your spatula under the half of the eggs without all the cheese and goodies and gently fold the plain half onto the filling. You want to cook the underside of the omelette to make sure the eggs are completely done and the fillings are all mixed together. Grab a large circular plat and gently plunk it on top of the omelette hemisphere. Place one hand on the pan handle and the other supporting the plate, and flip the omelette onto the plate. Slide it back into the pan, (upside down, now) and allow the omelette to cook for another 5ish minutes, until the eggs are lightly browned on both sides.
Serve with toast and tzatziki or sour cream to garnish.