Shred radishes, chop onions, grate ginger, and mince the garlic.
Combine vinegar, sugar, salt, coriander, cumin seed, and mustard seed in a large stainless steel or enameled dutch oven and bring to a boil.
Add the radishes, onion, ginger, and garlic. Return to a boil, stirring till all ingredients are heated through. Remove from heat.
Ladle relish into 4 hot half pint jars filling to 1/4 " of headspace between the top of the liquid and the lid. Remove air with plastic spatula and refill to 1/4 " if necessary.
Wipe the rims with a wet papertowel, add hot lids and rings. Process in water bath for 15 minutes, adjust for altitude.