Flat Omelet with Onions and Wine Vinegar

An omelette lyonnaise is made with lots of onions and is finished with a sprinkling of wine vinegar, a typically Lyonnaise trick that balances the onions’ sweetness.

Since cooking the onions takes about 45 minutes, I like to make this omelet for 4 people, in my 12-inch [30 cm] nonstick pan. I often serve it for lunch with plenty of red wine, most often Beaujolais. If you don’t have a 12-inch [30 cm] pan, make individual omelets—use 2 or 3 eggs each—in a 9-inch or 10-inch [23 or 25.5