Concord Grape Onion Jam

Immerse a jar with a tight fitting lid in simmering water until grape jam is ready. Wash lid in warm, soapy water.

Place grapes in a saucepan and crush with a potato masher until they begin to release some juice. Cover and cook over low heat until grapes are very soft and can be easily crushed, about 10 minutes. Remove from heat and set aside to cool.

Push grapes and all of the accumulated juice through a food mill or a fine mesh strainer. Extract as much pulp and juice as you can and return back to the saucepan. This should yield a generous 2 cups pulp. Discard leftover seeds.

Add onion, brown sugar, and balsamic vinegar to grape pulp and mix until sugar has dissolved. Bring to a boil over medium-high heat, then reduce temperature to low and simmer, uncovered, stirring occasionally at the beginning and then more regularly to prevent scorching, until jam is thick, about 30 minutes. (Do not overcook--jam will thicken more as it cools.)

Pour hot jam into the sterilized jar, seal tightly with the clean lid, and allow to cool. Store in the refrigerator and use within 2 weeks.