Preheat grill to medium-high. Grill lemons, cut sides down, until marked, 5 minutes. Brush onions with oil, and grill, flipping once, until soft, about 5 minutes per side. Grill rosemary until fragrant, 20 seconds.
Remove rosemary leaves from sprigs, and toss with juice from 2 grilled lemons, onions, oil, salt, and pepper.
Season pork chops with salt and pepper, and coat with flour, shaking off excess. Grill chops on the hottest part of the grill, flipping once, until marked, 1 to 2 minutes per side. Move chops to edge of grill, and cover. Cook, flipping once, until a meat thermometer registers 145 degrees for medium, about 2 minutes per side. Transfer to a plate. Let stand, covered, 10 minutes. Serve with remaining lemons and onion mixture.