In a large skillet over medium heat, add butter & saute onions, stirring often, until they are slightly translucent, about 5 minutes. Add apple wedges & thyme; cook until apples are tender. Add 1/3 of the quince sauce, tossing to coat both onions & apples well then simmer for about 1 minute. On each serving plate, place some apples & onions & top with a roasted pork chop. Serve with remaining quince sauce.