Onion puree is made by processing or mashing onions until a paste or thick soup is formed. The methods used to make onion puree range from very simple, in which the onions are boiled and then put through a food processor, to very complex, with the onions and additional spices cooked for hours before finally being pureed. Completed onion puree can be used as a spice in cooking, as a condiment for meats, as a side dish, or it can be converted into a main meal that resembles a soup or stew. In some types of Asian cuisine, including Indian cooking, onion puree often is used because it has very little texture and quickly can be browned in a pan once pureed. One way to keep onion puree is to combine it with garlic or a mix of garlic and ginger and form it into frozen cubes or blocks for later use.
Not all onion puree needs to be cooked before being run through a blender, mortar and pestle or food mill. Raw onions can be chopped into small pieces and combined with some type of liquid — water, white wine or olive oil are common — and then processed until a smooth paste forms. Puree that is made from raw onions is very pungent but can caramelize quickly in a hot pan. In this form, the puree freezes and refrigerates easily for storage.
A more flavorful type of onion puree can be made by combining onions with black pepper, basil, thyme, rosemary, fennel and savory in a heavy-bottomed pan. White wine, vinegar, beef stock, garlic, cloves and red pepper flakes sometimes also are added. The ingredients usually are sauteed in butter until aromatic, after which some water or other liquids are added. The whole mixture is allowed to cook slowly for several hours so the tastes merge, the onions become as soft as possible, and most of the liquid has reduced. Once completed, the mixture is pureed and, depending on the ingredients and the desired texture, may be poured through a sieve to remove larger particles.
Variations on onion puree use related aromatics to complement the onion flavor; these include shallots, garlic and green onions. The onions also can be smoked, grilled, caramelized or roasted before being cooked and pureed to get a different overall taste for the dish. The puree can be made very creamy by adding some type of thickener, such as cornstarch, corn meal, heavy cream or cooked flour and butter.