In a large ceramic bowl combine green tomatoes and onion. Sprinkle with salt. Refrigerate overnight.
Place cold water over tomatoes and onions and let stand i hour. Place black peppercorns, cloves, allspice, mustard seed, and ground mustard in a small cheesecloth bag.
In a large pot stir together vinegar and sugar and then add the tomatoes and onions, and the cheesecloth bag with the spices. Slice lemon mince red peppers, bring to a boil and simmer over low heat for 30 minutes.
Fill sterilized jars with tomato mixture, leaving 1/2 inch head space. Garnish with thin strips of red peppers. Arrange vertically alongside of jars. Screw on lid and process in a boiling water bath for 15 minutes.