Combine the vinegar, sugar, ginger, dry mustard, turmeric, mustard seeds, celery seeds, pickling spice and 1 teaspoon salt in a large pot. Bring to a boil over high heat, then reduce the heat and simmer 15 minutes to extract the flavors of the spices.
Return to high heat; once the mixture is boiling and steaming, add the tomatoes and onions, stirring to coat everything evenly. Reduce the heat and simmer 5 minutes. Remove from the heat and cool to room temperature.