Put the quince, single layer, in a baking dish and pour the syrup over it. Cook for about 2 hours, until it’s tender and bright ruby red, and the top bits are browned. Cool to barely warm, then serve or refrigerate, covered, for up to 5 days. Your syrup will probably set into a jelly when cold, so just reheat in the steam oven or very gently on the stovetop to bring it back to a liquid before serving.