Thoroughly clean and wash the fiddlehead and trim the stems to length.

In a clean 8 quart pan (enamel or stainless steel), mix fiddleheads, onions, salt and enough cold water to cover the fiddleheads and onions until the salt dissolves.

Add enough ice to cover the fiddleheads and onions.  Cover the container and place in the refrigerator for 2 hours.

Place the fiddleheads and onions in a colander and rinse well with cold water. Drain them thoroughly.

In an 8 quart saucepan, combine vinegar, sugar, mustard seed, celery seed and turmeric. Bring the mixture to a boil over high heat.

Add the fiddleheads and onions to the brine mixture and bring back to a boil.

Let them boil for 3 minutes, then spoon the fiddleheads and onions into clean sterilized canning jars, leaving 1/2 inch of headspace in the jar. Immediately spoon the hot brine over the fiddleheads.

Take a clean table knife and run it around the inside of the jar to remove any air bubbles.

With a clean cloth, wipe the rim of the jar and then put the 2 piece lids on the jars and tighten to fingertip tight.  Do not over tighten.

Place the jars in a water bath canner making sure that there is at least 1 inch of water over the canning jars. Heat to boiling.  Once the canner comes to a rolling boil, time it for 15 minutes.

Remove the jars from the canner and let them cool.  You will here a "pop" as the jars cool down. The lids should be slightly depressed once they are cool if the lids are seated properly.