Pat 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 ½ 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 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 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°), 8–12 minutes. Transfer to cutting board and let rest 10–15 minutes.
Cut chops. 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.