Finely chop cucumbers and onions. Place in a large bowl and sprinkle with salt, stir well. Let stand for 1 hour.
Drain mixture in a colander, rinse under cold water and drain again, pressing out any excess moisture.
In a large pot, bring vinegar, sugar, garlic, dill seed, mustard seed, celery seed to a boil.
Add cucumber mixture, bring to a boil again, stirring frequently. Reduce heat and simmer for 10 minutes. After 9 minutes, mix the cornstarch with a bit of the simmering juice until no lumps remain. Add to the relish and mix well until thickened.
Remove from heat, add the turmeric and mix well.
Pour into sterilized jars, leaving 1/2 inch head space. Seal with lids and rings. Process in boiling water bath in canner for 10 minutes.