Cut the green tomatoes, onions, and peppers into wedges.

In a food processor, cover and process the vegetables in batches until they are finely chopped.

Stir in the canning salt with the chopped vegetables.

Divide the vegetable mixture between two strainers, placing each over a bowl. Let it stand for 3 hours to allow excess liquid to drain. Discard the liquid from the bowls.

In a stockpot, combine the drained vegetables with the cider vinegar, sugar, celery seed, and mustard seed. Bring the mixture to a boil.

Reduce the heat and simmer uncovered for about 30 to 35 minutes, or until thickened.

Carefully ladle the hot mixture into eight hot pint jars, ensuring to leave 1/2 inch of headspace. Remove air bubbles and adjust the headspace if necessary by adding more hot mixture.

Wipe the rims of the jars clean and center the lids on them. Screw on the bands until they are fingertip tight.

Place the jars into a canner filled with simmering water, making sure they are completely covered. Bring to a boil and process for 15 minutes.

Carefully remove the jars from the canner and let them cool.