Make the brine: Combine 1 cup water, the gin, vermouth, 1/2 cup each sugar and salt, the rosemary, bay leaves and coriander in a large saucepan and bring to a boil. Remove from the heat and stir in 6 cups water and 2 cups ice; let cool to room temperature. Pour the brine over the pork chops in a large bowl, cover and refrigerate at least 4 hours or overnight.
Meanwhile, make the chutney: Combine the red onion, the remaining 1/2 cup sugar, the vinegar, mustard seeds, ginger, red pepper flakes and a pinch of salt in a medium saucepan and bring to a simmer over medium heat. Cook 3 minutes, then add half of the plums and cook, stirring occasionally, until the sauce thickens and the plums are tender, about 10 minutes. Add the remaining plums and cook until the first batch of plums starts falling apart and the second batch is just tender, about 8 more minutes. Stir in the mustard and let cool to room temperature. Preheat a grill to medium high. Drain the pork chops, discarding the brine, and pat dry. Sprinkle with pepper and grill, turning once, until grill marks appear and a thermometer inserted into the thickest part registers 145 degrees F, 6 to 8 minutes per side. Remove from the grill and let rest 5 minutes. Serve with the chutney. You can make the plum chutney up to 2 days ahead. Cover and refrigerate, then bring to room temperature before serving. Photograph by Kana Okada