Wash and dry the squash, zucchini and peppers.

Cut off the stems and slice the squash and zucchini into large chunks, discarding the seeds.

Remove the stems, seeds and veins from the bell peppers and cut into chunks.

Peel the onions and cut into chunks.

Working in batches, add the vegetables to a food processor, processing until finely chopped. 

Place the chopped vegetables in a bowl.

Sprinkle salt over the vegetables and stir to combine.

Cover and set aside for 1 hour. 

Thoroughly rinse the vegetables and drain well. Squeeze out excess liquid from the chopped vegetables. It's best to do this over the sink using your hands. Alternately, you can place the vegetables in a cheesecloth or flour sack towel to squeeze out the liquid over the sink. 

Place the drained vegetables in a bowl and set aside while you prepare the brine. 

In a large dutch oven, add the sugar, mustard seeds, celery seeds and black pepper. 

Mix in apple cider vinegar and place over high heat. Bring the mixture to a full boil, stirring often. 

Spoon in the chopped vegetables and return to a boil, stirring occasionally.

Lower to simmer, cook for 15 minutes, stirring often. 

Remove from heat. 

Spoon into sterilized canning jars. 

Wipe the tops of the jars and add sterilized lids and rings. Do not over tighten.

Prepare a large canning pot with boiling water.

Carefully lower jars into hot simmering water, making sure the jars are covered completely with water.

Bring to a boil. Cover and processing for 15 minutes. 

Carefully remove the jars and allow to cool to room temperature. 

Tighten the lids and store in a cool pantry cabinet up to 1 year.