Method
Slice the tomatoes, cutting away the stalk ends, and slice the onions thinly. Place sliced tomatoes and onions in layers, sprinkle each layer well with salt and leave overnight. Next day, drain off the liquid and place the slices in a preserving pan with the spices and mustard, tied in a muslin bag. Cover with vinegar and add the sugar. Bring to the boil, then simmer gentlyy for 20 minutes. Place in hot, sterile jars and seal.