Cut tomatoes, onions and peppers into wedges. In a food processor, cover and process vegetables in batches until finely chopped. Stir in salt. Divide mixture between two strainers and place each over a bowl. Let stand 3 hours., Discard liquid from bowls. Place vegetables in a stockpot; stir in vinegar, sugar, celery seed and mustard seed. Bring to a boil. Reduce heat; simmer, uncovered, 30-35 minutes or until thickened. , Carefully ladle hot mixture into eight hot pint jars, leaving 1/2-in. headspace. Remove air bubbles and adjust headspace, if necessary, by adding hot mixture. Wipe rims. Center lids on jars; screw on bands until fingertip tight. , Place jars into canner with simmering water, ensuring that they are completely covered with water. Bring to a boil; process for 15 minutes. Remove jars and cool.