Combine shredded zucchini and chopped onion in a large, non-metal bowl, and sprinkle with pickling salt. Cover, and refrigerate overnight.
The following day, prepare the boiling water canner. Heat jars and lids in simmering water until ready for use. Do not boil. Set bands aside.
Drain the zucchini in a colander, and rinse well with cool water. Squeeze out excess water and set aside.
In a large pot over medium-high heat, add the red and green bell peppers, sugar, vinegar, cornstarch, dry mustard, turmeric, celery seed, mustard seed and pepper. Stir to combine, then add the drained zucchini. Bring to a boil, then reduce the heat to medium-low, and simmer 30 minutes.
Immediately pack the relish into hot jars leaving 1/4-inch headspace. Remove air bubbles and wipe the rims. Center lids on jars. Apply bands and adjust to fingertip tight.
Process jars in a boiling water canner for 10 minutes (start timing when the water returns to boil) adjusting for altitude. Remove jars and cool. Check lids for seal after 24 hours. Lid should not flex up and down when center is pressed.