EASTERN NORTH CAROLINA BBQ SAUCE

2 cups cider vinegar
1 tablespoon peppercorns
1 teaspoon celery seed
1 teaspoon salt
1 tablespoon red pepper flakes
1 onion, finely chopped
1 cup water

Combine in a large saucepan. Bring to a boil, reduce heat, and simmer uncovered for an hour. Strain to remove the peppercorns if you like.

Adapted from source: Southern Food by John Egerton