Jalapeño-Marinated Grilled Pork Chops

Prepare a grill for medium-high heat; lightly oil grate. Pat pork chops dry and season generously all over with salt and pepper. Place on a rimmed baking sheet and let sit at room temperature at least 30 minutes and up to 1 hour.
Meanwhile, toast coriander seeds in a dry small skillet over medium heat, tossing often, until golden brown and fragrant, about 2 minutes. Transfer seeds to a cutting board and let cool. Using a flat-bottomed mug or a heavy skillet, lightly crush. Place in a small bowl and add vinegar, sugar, and 1/2 cup oil. Season with salt and pepper and whisk until sugar and salt are dissolved. Set marinade aside.
Toss jalapeños and 1 Tbsp. oil in a small bowl to coat; season with salt and pepper. Pat pork chops dry again (the salt will have drawn out more moisture) and rub with remaining 2 Tbsp. oil. Grill jalapeños, turning often, until softened and blackened in spots, about 5 minutes. Transfer to a cutting board. Grill pork chops, turning every 2 minutes or so, until cooked through but still medium-rare closest to the bone (an instant-read thermometer inserted near the bone should register 145°F), 8–12 minutes. Transfer to cutting board and let rest 10–15 minutes.
Cut pork along the bone to remove meat in one piece; slice 1/2" thick and transfer to a rimmed platter. Slice jalapeños crosswise into rounds and scatter over pork. Pour reserved marinade over and let sit at least 15 minutes and up to 1 hour before serving. To serve, scatter onion over.