In a food processor, process tomatoes, onions, and peppers in batches until finely chopped. (We found it easier to get more uniform chopping if we didn’t mix the vegetables together in the food processor.)

Combine vegetables in a large bowl and stir in canning salt. Divide the vegetable mixture in two large strainers, and place the strainers over a deep bowl to drain – being careful to avoid having the bottom of the strainer sit in the drained liquid.

Let the vegetables sit for about 3 hours to drain. If you’d like, use the back of a large spoon or the back of a smaller mesh strainer with handle, press on the vegetables and encourage them the drain more quickly.

Discard liquid that the vegetables released during the draining process. Pour drained vegetables in a large 12-quart stockpot. Add in vinegar, sugar, celery seed and mustard seed, then stir to combine.

Bring mixture to a boil. Reduce heat to a moderate simmer and cook uncovered for 30 to 35 minutes, stirring frequently. The mixture should be thickened with most of the liquids simmered off.

While the relish mixture is simmering, prepare a boiling water bath canner. Heat pint jars in simmering water until ready to use. Wash lids and bands, as well as a ladle and funnel with warm, soapy water. Set aside until ready to use.

Once the relish mixture has simmered and thickened, carefully ladle the hot relish into the prepared pint jars, leaving a ½-inch head space. Remove air bubbles and wipe rims clean. Place lids on the jars; screw on bands until finger tight.

Place jars in water bath canner with simmering water, making sure that the top of the jars are covered with water. Bring to a boil and process for 15 minutes. Carefully remove jars and cool on a rack – checking to make sure that the jars are sealed once cool.