- 2 tablespoons yellow mustard seeds (3/4 to 1 ounce)
- 3 3/4 pounds firm but ripe plums, halved, pitted, cut into 1/4-inch-thick slices
- 2 cups (packed) golden brown sugar (13 to 14 ounces)
- 2 cups distilled white vinegar
- 1/2 small white onion, halved, cut into 1/4-inch-thick slices (about 1/2 cup)
- 5 cinnamon sticks, broken into 1- to 1 1/2-inch pieces
- 1 tablespoon coarse kosher salt
- Stir mustard seeds in small dry skillet over medium heat until seeds darken slightly and begin to pop, about 5 minutes. Transfer seeds to heavy large saucepan. Add plums, brown sugar, vinegar, onion, cinnamon sticks, and coarse salt. Stir over medium heat until sugar dissolves. Increase heat to medium-high and bring to boil. Reduce heat to medium-low and simmer until liquid is thick and syrupy and mixture is reduced to 7 cups, stirring frequently, about 55 minutes.
- Ladle mixture into 7 hot clean 1/2-pint glass canning jars, leaving 1/2-inch space at top of jars. Remove air bubbles. Wipe jar threads and rims with clean damp cloth. Cover with hot lids; apply screw bands. Process jars in pot of boiling water 15 minutes. Cool jars completely. Store in cool dark place up to 1 year.