The one variable that will affect the cooking time is the moisture content of the onions. Unfortunately, I don't know of any way to tell that in advance of cutting into the onions. Fortunately, whatever the moisture level, you can adjust for it. For this method, it's great if your onions are high in moisture, because the water is what keeps them from browning too quickly or even burning.
Yield: About 2/3 cup
- 3-5 large onions (about 2 pounds)
- 2-3 tablespoons butter or oil
- Kosher salt
- 2-3 tablespoons dry sherry (optional)
2. Place the pan over low heat. Add the butter or oil to the pan and heat until butter is melted or the oil is shimmering. Add the onions and sprinkle generously with salt (about 1 teaspoon). Stir them and cover the pan. Cook on lowest heat, covered, for 15 minutes. Remove the lid and check the onions. They should have begun to give up moisture by this point, so there should be a shallow layer of onion juice and water in the pan. If not, add enough water to cover the bottom of the pan.
3. Cover and continue to cook, stirring and checking them every 15 minutes or so. For the first 45 or 60 minutes, they should contain liquid; if they're dry, add more water. The onions will soften, then at about the hour to 90-minute mark, they'll slowly start to turn golden and then amber in color. They'll lose most of their volume and continue to darken, until you should end up with about 2/3 of cup of deep amber onions. (Be careful how dark you let them get; if they get very dark brown, you may end up with bitter flavors.) If they're very high in moisture, you may need to remove the lid for the last 20 minutes or so of cooking to evaporate the remaining liquid. The total cooking time will probably be between 2 and 3 hours; keep in mind that the larger the volume of onions, the longer the cooking time will be.
4. If you like, turn up the heat to medium and add the sherry. Scrape up any browned onion bits that may be stuck to the pan and let most of the sherry evaporate. Let cool if not using immediately. The onions can be refrigerated for up to a week or frozen for several months.