Prepare a medium-high fire on a gas or charcoal grill. In a medium bowl, toss the pork chops with 1 tablespoon of the oil, half of the thyme, 1 teaspoon salt, and 1/2 teaspoon pepper. Put the onions on a large plate and sprinkle with 2 tablespoons of the oil and 1-1/2 teaspoons salt. In a small bowl, whisk the vinegar, sugar, and the remaining thyme.

Grill the onions, covered, flipping once, until crisp-tender, 5 to 6 minutes per side. Return to the plate. Grill the pork, covered, flipping once, until firm to the touch and just cooked through, 3 to 4 minutes per side.

Transfer the pork to a large platter, brush with some of the vinegar mixture and tent loosely with foil. Heat the remaining 1 tablespoon oil in a large skillet over medium-high heat until it's shimmering hot. Add the onions and the remaining vinegar mixture and cook, stirring, until they absorb all of the liquid and take on a browned, glazed appearance, 3 to 4 minutes. Serve the pork chops with the onions.