The most famous mostarde come from Lombardy in Italy's north: from Cremona, where whole, mixed fruits are candied, spiced and eaten with boiled meats; and Mantova (Mantua), where spiced, barely-cooked quince or apple is eaten with cheeses. Mostarda mantovana, has been made essentially the same way for centuries. I recently met with Italian restauranteur, Anna Maria Eoclidi, whose restaurant, Pasta Emilia (http://emilia.com.au/), is a little gem of northern Italy in Sydney, Australia—this is more or less how she recounted her aunt's recipe to me, except that her aunt uses apples rather than the more old fashioned quince. (I prefer quince for its color, but as they are so seasonal, apples are easier to come by.) When choosing fruit for this recipe, always go for slightly less ripe fruit.
In Italy, the mustard essence (not to be confused with mustard oil) is bought in a pharmacy, not a food store. It comes in a tiny bottle and is sold by the drop—you only need about 12 potent drops per kilogram of cooked fruit—and not without reason does the bottle have a skull and crossbones on it. It's an extremely dangerous, volatile essence when not used carefully. You must use it in a well-ventilated place, and just by looking at it, you'll feel your eyes burn and your nose tingle. Luckily those few drops go a long way and, rather like the prized scent of truffles, it must be treated carefully. Place it in the fruit only once cooled, then close the jars right away. You can begin eating the mostarda immediately but don't wait too long—about 6 months after bottling, the distinct, intense mustard scent begins to lose its punch.
While this recipe is a 4-day process, the active time is a scant 30 minutes.
Outside of Italy, it is probably difficult, if not impossible, to find. But there are other ways to replicate a similar mostarda (and perhaps you can save the real deal experience for your next trip to Italy). You can do a similar thing by using mustard powder and warmed white wine or dessert wine in place of the mustard essence. Here on Food52, contributor Maria Teresa Jorge uses 2 ounces of white mustard powder diluted in 1/4 cup of vinegar to replace 10 drops of mustard essence (https://food52.com/recipes...), and we even have a no-recipe guide for the condiment (https://food52.com/blog...).
Featured in: The Italian Condiment Your Cheese Plate Needs Pronto. —Emiko
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