4 medium red and/or yellow onions, halved, stem ends intact
5 tablespoons olive oil, divided
Bring brown sugar, 1 cup salt, 2 bay leaves, 1 Tbsp. coriander seeds, 1 tsp. peppercorns, and 2 cups water to a boil in a medium saucepan. Reduce heat and simmer, stirring occasionally, until sugar and salt dissolve, about 4 minutes. Transfer brine to a large bowl and add cider and 2 cups ice. Let cool.
Place pork and brine in a large (2-gal.) resealable plastic bag. Seal bag and chill at least 8 hours.
Remove pork from brine, rinse and pat dry with paper towels. Let sit at room temperature 1 hour.
Toss potatoes and onions with 4 Tbsp. oil in a large roasting pan or on a large rimmed baking sheet; season with salt and pepper and set aside.
Place rack in lower third of oven and preheat to 425°. Using the side of a chef’s knife, crush remaining 2 Tbsp. coriander seeds. Season pork with salt and pepper and rub all over with crushed coriander. Heat remaining 1 Tbsp. oil in a large skillet over medium-high heat. Cook pork until browned on all sides, 8–10 minutes.
Transfer pork to roasting pan and nestle in among vegetables. Tuck remaining 6 bay leaves under kitchen twine over top of pork. Wrap bone tips with foil to prevent burning and roast, turning vegetables halfway through, until an instant-read thermometer inserted in the center of pork registers 140°, 60–75 minutes.
Remove foil from bones and transfer pork to a cutting board; let rest 30 minutes before slicing between ribs into chops.