Onions, dehydrated are made from fresh yellow onions which have been chopped then dehydrated. Commercially dehydrated onions are used in dry soup bases and other dried meal products. At home they can be used when fresh onion is unavailable and can be added to any recipe including dips, soups, or sauces.
This is a product found in the spice section of most well-stocked grocery store. You can optionally purchase them online at Amazon.com or Walmart.com.