Combine onion and peppers in a large glass, ceramic, or stainless steel container. Cover with boiling water; let stand 10 minutes. Drain thoroughly.
Combine 2 2/3 cups vinegar and water in a medium-size stainless steel saucepan; bring to a boil. Pour over pepper mixture; let stand 10 minutes. Drain thoroughly.
Combine remaining vinegar, sugar, mustard and celery seeds, and salt in saucepan; bring to a boil. Add drained pepper mixture; simmer 2 hours or until onions are translucent, stirring frequently.
Pack hash into hot sterilized jars, leaving 1/3-inch head space. Remove air bubbles. Cover with metal lids, and screw bands tight. Process in boiling-water bath 20 minutes. Serve with meats or vegetables.