In a grinder or food processor, coarsely grind tomatoes, red bell peppers, green bell peppers, and onions. Line a large colander with cheesecloth, place in sink or in a large bowl, and pour in tomato mixture to drain for 1 hour. In a large, non-aluminum stockpot, combine tomato mixture, celery seed, mustard seed, salt, sugar, and vinegar. Bring to a boil and simmer over low heat 5 minutes, stirring frequently. Pack relish into sterilized jars, making sure there are no spaces or air pockets.
Fill jars all the way to top.
Screw on lids. 4 Place a rack in the bottom of a large stockpot and fill halfway with boiling water. Carefully lower jars into pot using a holder. Leave a 2 inch space between jars. Pour in more boiling water if necessary, until tops of jars are covered by 2 inches of water.