I just brewed up a small batch of onion wine. At present, pre-fermentation, pre-yeast addition, pre-miraculous physical changes, it tastes sort of gross. Really, really sweet.
The recipe that I'm using is this one, though I was originally turned on to the idea by fellow fermentation fiend and friend Mike, who owns some strange pamphlet-like book that is chock-full-o' interesting wine recipes. This one in particular is essentially made of the water from boiled onion and potato, which is then mixed with chopped raisins, sugar, and some random wine additives (acid mixture, yeast energizer, campden tablets). And, of course, yeast, which is the magic ingredient.
In any sort of fermentation like this, yeast is in fact a non-essential ingredient, as the natural yeasts present in the air will invade the liquid and ferment it. Most brewers and winemakers, however, tend to use to store-bought yeast, in order to make the final product somewhat more predictable. The campden tablets mentioned above apparently work to inhibit random wild yeasts from taking over the process.
My fermentation hero, and self-described "fermentation fetishist", Sandor Katz, is the type of guy who does not use store-bought yeast, but rather relishes the natural, unpredictable, and eminently local method of "wild fermentation" (also the name of one of his books).
I'll keep you all posted on how this onion wine comes out. From what I've read, it is supposed to be quite good, and not taste like onions at all. We'll see.