Sprinkle pork chops with salt and pepper. Cook coriander and mustard seeds in a medium saucepan in 1 tsp. oil over high heat until mustard seeds begin to pop, 1 minute.
Remove from heat and stir in ginger, vinegar, and brown sugar; set aside.
Grill pork chops, turning once, until cooked medium, about 10 minutes total.
Brush plums and onion with remaining 2 tsp. oil. Grill onion, turning once, until softened and slightly charred, 8 to 10 minutes. Grill plums, turning once, until grill marks appear, 3 minutes.
Transfer onion and plums to a cutting board and cut pieces in half. Toss in pan with reserved spice mixture and serve over pork chops.
