1/2small onion, finely chopped(about 1/3 cup chopped)
1tablespoon finely chopped green pepper(such as jalapeño or serrano, or a milder poblano if you don't like the heat)
1tablespoon whole yellow mustard seeds
1/2tablespoon kosher salt
1garlic clove, minced
1/2tablespoon finely chopped fresh ginger
Instructions
Place pitted plums in the bowl of a food processor; pulse briefly until coarsely chopped (do not purée). You can also chop the plums by hand if you prefer a coarser texture. You should end up with about 5 cups of chopped plums.
In a large stainless-steel saucepan, combine brown sugar, sugar, vinegar, onion, pepper, mustard seeds, salt, garlic and ginger. Bring to a boil over high heat, stirring constantly. Add chopped plums and return to a boil. Reduce heat to medium and boil gently, stirring occasionally to prevent scorching, until sauce is thick and syrupy, about 1 1/2 hours.
While sauce is cooking, prepare canner and wash/sterilize 4 half-pint (or equivalent) canning jars. Keep jars in hot (not boiling) water until ready to use.
Ladle hot jam into jars, leaving 3/8-inch of headspace. Remove any air bubbles and adjust headspace, if necessary. Wipe the rims clean and top with lid; screw on ring until finger tight. Process in a boiling water bath for 15 minutes. Remove from water and let cool completely, 12 to 24 hours. Check seals.
Serve with pork, as dipping sauce for egg rolls, spread over scallion pancakes or served with moo shu pork.
Notes
If you don’t want to go through the hassle of canning the plum sauce, ladle into jars and use within 2-4 weeks.