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...).
Day one: Peel and finely chop the fruit. Place in a non-reactive bowl with the sugar and lemon slices (if using) and toss until well-combined. Leave to sit in a cool place to macerate for 24 hours.
Day two: Strain the liquid created by the fruit into a small saucepan. Return the fruit to the bowl. Bring the liquid to a rapid boil, cook for 5 minutes. Remove from the heat and pour the liquid over the fruit. Let cool, then cover and let sit in a cool place for another 24 hours.
Day three: Strain off the liquid again, into a saucepan. Return the fruit to the bowl. Bring the liquid, which should be more syrupy now, to the boil and cook 5 minutes. Pour it back over the fruit. Let cool, then cover and let sit in a cool place for 24 hours.
Day four: Place the fruit and its syrup in a saucepan and bring to the boil. Turn the heat down to a simmer and let cook, turning the fruit with a wooden spoon to cook it evenly, for about 10 minutes (if there is not enough liquid left to cook the fruit through, add a splash of water to help it along). The fruit should still have some bite to it, it should keep its shape, but be evenly cooked. It should all be held together with a light syrup.
Let it cool completely, then, weigh the fruit (you do need scales for this! In my experience with this recipe, it produces about 750 grams of cooked fruit, so 3/4 of a kilogram). In a well-ventilated area, open the bottle of mustard essence (do not put your face close to it or try to sniff it directly and do not try to taste it or get it on your skin!) and carefully add 10-20 drops of mustard essence per kilogram of fruit or to taste—10 drops will produce a mildy spiced mostarda, 20 is not for the faint hearted! I would recommend 10-12 drops per kilogram (so 8-9 drops for 3/4 of a kilogram of cooked fruit). Bottle in sterilised jars and, if not planning on eating within the month, process the cans to seal. This is best consumed within six months as the mustard essence begins to weaken significantly after this time, even if unopened. As the essence diminishes with heat, it is best to keep the mostarda somewhere cool.
The Australian-Japanese cookbook author has lived in Florence (where a visit to a cheese farm once inspired her to start a food blog) for over 10 years with her Tuscan sommelier husband and two kids. Her third cookbook, Tortellini at Midnight, is out now.