Sweating is a technique you may be performing in the kitchen already without even realizing it. Whenever you make a soup, stew, or stock for instance, you have probably allowed the onions, celery, and carrots to sweat before adding the remaining ingredients. Sweating, simply, is cooking vegetables with a small amount of fat over low heat until they are tender.
Unlike caramelizing, when you sweat your vegetables you're not cooking them over a high heat or looking for that golden color. For instance, when you sweat onions, the end result is tender, translucent, and evenly cooked. Crisp vegetables are the best candidates for this technique because sweating helps develop the flavor and sweetness of the vegetables while ensuring they will not stand out in the finished product. You want the flavor of those carrots in your chicken noodle soup, but you don’t want to crunch on them or feel like they are overpowering all the other wonderful flavors already in the dish.
When you're sweating your vegetables, make sure that your pan is not too hot. Add butter or oil to a pan heated at medium-low heat. Once warm, add vegetables and a pinch of salt. You're looking for an even and light sizzle not a bunch of pops. Adjust the heat as needed and continue stirring to ensure your veggies don’t caramelize or brown. Once they're tender, after about 10 minutes, they're ready! Remember to cut your veggies into equal sizes (a 1/2 inch dice works well) for even cooking and if you want to add garlic do so about half way through the process (to ensure the garlic doesn't brown or burn) before your veggies are ready.
Ahora, get to sweating for sopas sake!