Place coarse chopped cucumbers into food processor and pulse until finely chopped. Measure six cups of chopped cucumbers and place in a large bowl.
Place coarse chopped sweet peppers, seeds and membranes removed, into food processor and pulse until finely chopped. Measure 2 cups of chopped peppers and add to cucumbers.
Place coarse chopped onions to processor and pulse until finely chopped. Measure 2 cups of chopped onions and add to cucumbers.
Add kosher salt to vegetables and stir to combine well.
Cover with ice water and set aside for 2 hours.
Drain in a colander, rinse well and drain thoroughly. Squeeze out excess liquid if necessary
Combine sugar, vinegar, celery seed and mustard seed in a large stock pot and bring to a boil.
Stir in drained vegetables and simmer for 10 minutes.
Spoon hot relish into hot pint jars, leaving 1/4 inch space.
Top with lids and rings, just hand tight.
Place jars into boiling water and process for 15 minutes.
Remove jars and set aside to cool completely.