In a large ceramic bowl or crock, combine green tomatoes and onions. Sprinkle with salt. Refrigerate overnight.
Pour cold water over tomatoes and onions, and let stand for 1 hour. Place peppercorns, cloves, allspice berries, mustard seeds, and ground mustard in a small cheesecloth bag.
Drain tomatoes and onions.
In a large pot, stir together vinegar and sugar, and then add the tomatoes and onions, the cheesecloth bag with spices, the sliced lemon, and minced red pepper. Bring to a low boil, and then simmer over low heat for 30 minutes. Check frequently; you may need to pull the pot off of the heat occasionally to prevent scorching. Discard spice bag.
Fill sterilized jars with tomato mixture, leaving 1/2 inch headspace. Garnish with thin strips of red pepper arrange vertically along the sides of the jars. Screw on lids, and process in a boiling water bath for 15 minutes. Remove jars and let jars cool completely. Refrigerate any that haven’t sealed completely. Store sealed jars in a cool dark place.