Onion Omelet with Vinegar and Walnuts

If I had to name favorites, this is the omelet I’d choose. I love the sweet, delicate bite of the onions and the lively finish of butter and vinegar, reduced at the last minute to syrupy froth. A teaspoon or so of walnut oil mixed with the onions gives them a rich, nutty flavor, as do the walnuts themselves. This makes a good fall meal, served with an escarole salad and a piece of Gruyère cheese. With such a substantial filling, this omelet works best cooked in a large nonstick pan and fini