Chinese-Style Plum Sauce

From the garden

• 4 lb ripe plums, pitted and roughly chopped, skins on · any type; red-fleshed plums give the deepest color
• 1 medium onion, finely chopped
• 3 cloves garlic, minced

From the pantry

• 2 tbsp fresh ginger, peeled and grated
• 1 cup brown sugar, packed
• ½ cup sugar
• 1 cup apple cider vinegar · 5% acidity; do not reduce this amount
• 1 tsp red pepper flakes
• ½ tsp ground cinnamon
• ¼ tsp ground cloves
• 1 tsp mustard seed (optional)
• 1 tsp salt

1. Fill a boiling-water canner and bring it to a simmer. Wash 6 half-pint jars and keep them hot in the simmering water; prepare the lids per the manufacturer's directions.
2. Combine the plums, onion, garlic, ginger, both sugars, vinegar, pepper flakes, cinnamon, cloves, mustard seed, and salt in a wide, heavy, non-reactive pot. Bring to a boil over medium-high, stirring to dissolve the sugar.
3. Lower the heat and simmer, uncovered, stirring often, 45–60 minutes, until the plums have completely collapsed and the mixture has thickened to the consistency of applesauce. Stir more frequently toward the end so the bottom does not scorch.
4. For a smooth sauce, puree with an immersion blender right in the pot, or work through a food mill to remove the skins. Return to a boil for 2–3 minutes; it should mound softly on a spoon and leave a trail when you drag a spatula across the pot. Taste: it should be sharp, sweet, and warmly spiced.
5. Ladle the hot sauce into the hot jars leaving 1/4 inch headspace. Wipe the rims, center the lids, and screw the bands on fingertip-tight.
6. Process in the boiling-water canner 15 minutes (start timing when the water returns to a full boil; add 5 minutes above 1,000 feet, 10 minutes above 3,000). Turn off the heat, wait 5 minutes, then lift the jars out and cool undisturbed 12–24 hours. Check that the lids have sealed; refrigerate any that have not.