Dried white onion slices are one of the best shortcuts you can use to save time in the kitchen. Add these slices of onion to dishes in marinades and brines or those with long cooking times. To rehydrate, simply soak in warm water for about 15 minutes, then drain. Once they’re rehydrated, they’re a flavorful substitute for fresh onions. Five teaspoons dehydrated minced onion equals about 1/4 cup rehydrated onion. Ingredients: Onion.