Put the sliced tomatoes in the stoneware crock, sprinkling each layer with salt. Let stand overnight and drain off the juice. Chop the tomatoes roughly. Add to the heavy-bottomed saucepan along with the onions, mustard, pepper, ginger, allspice, red pepper flakes, and brown sugar. Pour the vinegar over all, bring to a simmer and cook very gently for 1½ hours.
Sterilize the jars, their l