Making and canning zucchini relish is a great way to preserve the summer glut of zucchini from your vegetable garden.
Add the chopped zucchini, peppers, and onions to a large bowl.
Sprinkle salt over the vegetables and toss to combine. Cover with crushed ice or ice cubes and water and let stand for 2 hours.
Wash your jars and lids in warm, soapy water and rinse thoroughly.
Place jar rack into water bath canner, set jars in the canner, add water to cover jars, and boil jars for 10 minutes to sterilize.
Warm your lids in a small pot of water over low heat. Keep jars and lids warm until ready to use.
Drain the vegetables and rinse well to remove the extra salt.
Add the sugar, celery seed, mustard seed, and vinegar to a large pot. Bring the pot to a simmer and then add the drained vegetables to the pot. Stir to combine and simmer for 10 minutes.
Spread a kitchen towel on the counter. Use your jar lifter to remove the warm jars from canner. Drain and line up the jars on the towel.
Use the canning funnel and ladle the relish into the jars leaving 1/2-inch headspace.
Remove air bubbles with the bubble popper and wipe the rims.
Use the magnetic lid lifter to lift lids out of the warm water, center lid on the jar, and screw on band until it is fingertip tight.
Using jar lifter, place jars carefully into water bath canner on the jar rack leaving space in between them. Once jars are all in canner, adjust the water level so it is 2-inches over the tops of the jars. If adding water, use the hot water from your small pot.
Cover the canning pot and bring the water to a rolling boil over medium-high heat. Process the jars for 10 minutes. When processing time is complete, turn off heat and allow the canner to cool down for 5-minutes.
Spread a kitchen towel on the counter. Use a jar lifter to lift jars carefully from canner and place on the towel. Allow the jars to cool for 12 to 24-hours. You should hear the satisfactory "ping" of the jar lids sealing.
After 12 to 24-hours, check to be sure jar lids have sealed by pushing on the center of the lid. The lid should not pop up. If the lid flexes up and down, it did not seal. Refrigerate jar and use up within a few days.
Remove the screw on bands and wash the jars. Label and date the jars. Store in a cool, dark location. Allow 4-5 weeks for the relish to develop their flavor. Use within a year.