Onions which have been minced and dehydrated. Onion flakes can be used to replace fresh onion in soups, stews and other cooked foods. They can be used as an onion replacement in various recipes. To make onion flakes, onions are simply chopped and then allowed to dry, either in the sun, in a dehydrator, or in an oven on a low setting. As the onions dry out, they shrink dramatically.