1. Day 1: Peel the pears and quarter them lengthwise. Cut out the core and seeds. Slice the quarters into chunks, or cut them crosswise into thin slices. Place the pieces in a shallow bowl and add the sugar. Scrape the seeds from the vanilla bean and add them to the pears. Add the lemon juice and fold everything together gently so as not to break up the pears. Set a plate on top of the fruit and weight it down with a heavy object. Cover with plastic wrap and let the pears macerate overnight.
2. Day 2: Drain the pears in a colander set over a bowl to catch the syrup. Transfer the syrup to a heavy-bottomed saucepan and bring it to a boil over medium-high heat. Boil for 5 minutes, then add the pears. Raise the heat to high and boil the pears for 1 minute. Then remove from the heat and pour the pears and the syrup into a shallow heatproof bowl. Cover with a plate and weight it down with a heavy object. Cover with plastic wrap and leave overnight.
3. Day 3: Drain the pears in a colander set over a bowl to catch the syrup. Return the fruit to the heatproof bowl. Transfer the syrup to a heavy-bottomed saucepan and bring it to a boil over medium-high heat. Boil for 5 minutes. Pour the hot syrup over the pears. Set a plate on top and weight it down with a heavy object. Cover with plastic wrap and leave overnight.
4. Day 4: Repeat the process one more time, draining the fruit in a colander and boiling the syrup. The syrup should be thickened like maple syrup rather than honey. If it becomes too thick, add a splash of water and boil briefly.
5. At this point you need to determine whether the pears are sufficiently candied. Taste a piece. It should be dense, slightly sticky, and tender, and not raw at all. (If the pears still have an uncooked texture, pour the hot syrup over them, weight them down, cover with plastic, and leave overnight.)
6. Once they are properly candied, let the pears and syrup cool to room temperature. Wearing kitchen gloves, add about 1 teaspoonful of the mustard essence to the pears in syrup and stir to combine thoroughly. Taste to determine the spiciness, and add more as desired. I find that 1 tablespoon of mustard essence is about right, but I like it daringly spicy. Carefully spoon the mostarda into the jar. Store it in the refrigerator, where it will keep for at least 6 months.
Excerpted from Preserving Italy, © 2016 by Domenica Marchetti. Reproduced by permission of Houghton Mifflin Harcourt. All rights reserved.
"Vegetables are perishable, so we don't have any indication of what they looked like 500 years ago," says James Nienhuis, a professor in the Department of Horticulture at the University of Wisconsin.