Step 1Heat the oven to 375 degrees. Wrap the quince in foil and roast until tender, about 1 1/2 hours. Peel and core the fruit. While the quince is roasting, combine the honey and vinegar in a medium saucepan and cook over medium heat, stirring occasionally, until reduced by two-thirds, 15 to 20 minutes.
Step 2Add the quince and the honey-vinegar reduction to a blender or food processor and puree until smooth.
Step 3In a large sauce pot, bring salted water to a boil and add the Brussels sprouts. Cook until tender, about 5 to 8 minutes.
Step 4In a medium skillet, heat the duck fat or schmaltz over medium-low heat and cook the onions until soft and translucent, about 12 minutes. Remove from heat. Add the Brussels sprouts to the onions and enough quince puree to coat, tossing to combine (you may not use all of the quince puree). Season with one-half teaspoon salt, or to taste. Serve immediately.