Peel and quarter onions. Seed and quarter red peppers. Zucchini should be split lengthwise and seeds removed by scraping a small spoon down the center. Cut zucchini halves into 4″ chunks.
Working in batches place about 1/3 of each vegetable in the food processor and pulse until it is a coarse grate. Do not over process or you will have mush. Repeat with the remaining vegetables in 2 more batches.
Transfer chopped vegetables to a large bowl. Add the 1 Tbsp of salt and mix in. Cover and refrigerate overnight.
The next day rinse the vegetable mixture with cold water and drain well in a colander.
While the vegetables are draining heat the syrup ingredients in a large pot. |(Pot should be large enough to eventually hold the vegetable mixture and the syrup.) Stir the syrup until the sugar is dissolved. Then add the vegetable mixture and simmer 30 minutes.
While the mixture is simmering, sterilize your jars for 15 minutes and lids for 5 minutes.
Fill hot jars with hot zucchini mixture. Clean the rim of the jars and seal with the rubber lid and screw band. Do not over tighten the screw band- just finger tight.
Add enough water to your canning vessel to cover the sealed jars. Bring water to a boil, submerge the jars and simmer for 10 minutes in the water bath.
Remove jars with tongs and hot towels. Allow jars to come to room temperature away from drafts. When cool double check the screw bands. You may need to tighten some again. You will hear pops as the jars cool when the lid seal is suctioned to the jar rim.
Store jars in a cool, dark place up to one year. The seal should be tight when you go to open. If any are not sealed at the point of opening do not use them. (I have never had this actually happen.)