Sweating onions is a technique that lives up to its name. Cooking onions over low heat makes them release moisture and appear dewy, shiny or -- you guessed it -- sweaty. When you sweat onions, you try to cook them without their turning brown. Your may want to sweat onions to give them a head start in cooking before you add more veggies to the pan, or you may need onions that are soft but don't have the sweetness of caramelization. This technique is simple but requires some patience. Luckily, your kitchen will smell delicious.
Chop the onions into pieces all roughly the same size so they cook uniformly. Make the pieces any size as long as they're the same size.
Warm a skillet or saute pan over medium-low heat. Stainless steel, nonstick or any other type of pan works as long as the pan heats evenly.
Pour a few tablespoons of oil into the pan and let it heat for a minute. Transfer the onions to the pan. Add a pinch of salt, which will help draw liquid out of the onions. Stir the onions around until all the pieces are thinly coated with oil and evenly distributed in the pan.
Cover the pan with a lid or aluminum foil to keep the moisture trapped and help prevent the onions from browning. If you use aluminum foil, keep one edge of the pan uncovered so you can easily see if the onions start to caramelize.
Stir the onions around once a minute. Turn the heat down to low if the onions start to brown. They will start to sweat after a few minutes of cooking. Cooking them until they turn translucent generally takes from 10 to 20 minutes, depending on how full your pan is and the heat of your burner.
Remove the onions from the pan if you're not going to continue cooking with them right away. They will continue to cook and may start to brown if left in the hot pan.
You may add more flavor to your onions by sweating carrots, celery and fennel along with them, provided all ingredients are cut into pieces of the same size.