A recent lunch: une omelette au fromage, aux épinards, et aux champignons. There is no particular recipe, I guess, but there is a method. I usually make a five-egg omelet for the mid-day meal, and we usually have leftovers, which can become a nice, light supper. I cook the spinach (frozen) beforehand, seasoned with salt, pepper, and a grating of nutmeg, in the microwave, and I sauté the sliced mushrooms quickly, unsalted, in butter or a mixture of butter and oil, over high heat, so they don't poach in their own liquid.
A slightly runny, soft, tender omelet — not overcooked
One secret to making a nice puffy omelet is this: if you want to add a little liquid to the beaten eggs (les œufs battus en omelette), just add a little bit of water. Never add milk because the milk won't really blend into the eggs (a tip I learned from watching Jeff Smith, "The Frugal Gourmet"). The steam from water makes the eggs puff up slightly. When the omelet is just barely starting the brown in the pan, but the top of the omelet is still not set — not completely cooked — sprinkle on as much grated cheese as you like. Pepper the eggs but don't salt them yet. Make sure the omelet is not stuck to the bottom of the pan (non-stick coatings are great).
Thanks again to CHM for the gift of this nice set of stainless-steel serving dishes, a classic in France.
On the half of the omelet that is farthest from the handle of the frying pan, lay down the cooked spinach with the mushrooms on top it. Then tip the pan to start sliding the omelet out onto the serving dish. When about half or a little more of the omelet is out of the pan and is on the serving dish, lift up the pan to fold the "undressed" side of it over the filling ingredients. Voilà. The omelet is folded but the filling ingredients are still visible. Serve with French fries and a salad if that floats your boat.