I keep a small zip-top bag full of apple cores, peels, and other scraps in my freezer, and I add to it every time I cut an apple for a snack or make a batch of cider. When I have enough to pack into a canning jar, I turn those scraps into vinegar. Essentially, this works by mixing the scraps with water and sugar, letting it ferment, and then ignoring it until the liquid turns into vinegar. The resulting vinegar is slightly less intense and flavorful than “real” vinegar, but still perfectly tasty for making everything from salad dressing to tri-tip marinade.
Making mostarda is an art and a process. It requires poaching fruit—either whole, large pieces, or sliced—in syrup over the course of several days, until it is saturated and beautifully translucent. Once this candying process is completed, mustard essence is stirred in drop by drop until the mostarda is sufficiently spicy—spicy enough to make your eyes water! The type of fruit depends on what’s in season, but quince, pears, apples, melon, and figs are all good candidates. Of all those, I prefer pears, which take beautifully to the candying process without (by some miracle) turning mushy at all. Although traditional mostarda is made with small whole or halved fruits, I cut the pears into thin slices, as I find the large pieces unwieldy.