Pit and finely chop plums. Measure 10 cups, set aside. Place 6 clean pint canning jars on a rack in a boiling water canner; cover jars with water and heat to a simmer (180°F/82°C). Set screw bands aside. Keep jars and sealing discs hot until ready to use.
Combine sugars, vinegar, onion, mustard seed, chili peppers, salt, garlic and gingerroot in a large stainless steel saucepan.
Bring to a boil; add plums. Return mixture to a boil; boil gently, stirring occasionally, about 1 3/4 hours or until thick and syrupy.
Ladle sauce into a hot jar to within 1/2 inch of top of jar (headspace). Using nonmetallic utensil, remove air bubbles and adjust headspace, if required, by adding more sauce. Wipe jar rim removing any food residue.
Center hot lids on clean jar rim. Screw band down until resistance is met, then increase to fingertip tight. Return filled jar to rack in canner. When canner is filled, ensure that all jars are covered by at least one inch of water.
Cover canner and bring water to full rolling boil before starting to count processing time. Process boil filled jars 10 minutes.
Makes 6 pints. (you can also ladle into half pints to give as gifts for the holidays)