There are several ways to sauté without oil, but I recommend the method taught in the Forks Over Knives Cooking Class because it is the quickest. When you use this method, you will heat your pan to quite a high temperature. It might be counter-intuitive, but at a hot enough temperature, onions and other vegetables won’t stick much to a pan.
To determine whether your pan is hot enough, add an eighth of a teaspoon of water to your heated pan. If it is at the correct temperature, the water will form a ball that looks like a mercury ball. If your pan isn’t hot enough, the water will spread out or steam. If your pan is too hot, immediately upon adding the eighth teaspoon of water, many tiny balls of water will form in the pan.
This You Tube video shows how to judge the temperature of the pan. Just ignore the last part of the video when oil and chicken are added. This is the point when you can add the onions and other vegetables you would like to brown in the pan without adding any oil.
Once you add your onions, let them cook at least 15 seconds, and then stir. Repeat until the onions are brown. Because you are using a very hot pan, I recommend that you have a few tablespoons of water available in case your onions start to brown too quickly. If you are concerned that your onions may burn, add the water, stir, and remove the pan from the heat and allow to cool briefly before continuing your recipe.
If you have garlic to brown, add when your onions are nearly done. If you sauté the garlic too long, it will burn and taste bitter. To avoid this, add the garlic and stir with the onion just about 30 seconds, and then add some water or the ingredients in the next step of the recipe to cool down the pan somewhat.
Another thing to consider when sautéing is the pan’s material. I use stainless steel pans. If your pan has a non-stick coating, I don’t recommend heating it to such a high temperature because this may damage your pan. Please follow the manufacturer’s instructions instead. You can brown onions at lower temperatures without oil; the process will just take longer.