Apple Orchard Barbecue Sauce

1 medium onion, grated or finely chopped
4 cups pressed apple cider (not pasteurized apple juice)
1/2 cup cider vinegar
1/2 cup firmly packed dark brown sugar
1 tablespoon whole mustard seed
1 teaspoon celery salt or celery seed
1 tablespoon Worcestershire sauce
1 tablespoon liquid smoke
2 (12 ounce) bottles chili sauce
Hot pepper sauce, to taste (optional)

Bring all ingredients to boil in a large saucepan. Reduce heat and simmer until sauce is thickened and volume is halved. Pour hot sauce into two 1-pint sterilized jars. Seal with 2-piece sterilized lids. Water-bath process for 10 minutes. Remove with tongs. Cool.