You can turn garlic and onion peels into vegetable stock by simply stashing them in a freezer bag, and when it is filled, you'll have enough to make a stock, according to Food & Wine. Similar to cooking normal stock, to make vegetable stock from onion and garlic, add the peels to a pot of water with the peels, bring it to a boil, and simmer for at least 2 ½ hours.