1) Combine vinegar, caster sugar, dry mustard, turmeric, mustard seeds, celery seed, cayenne pepper, and ginger in a large pot and bring to a boil over high heat. Reduce heat and simmer for 5 to 10 minutes to essence the flavors of the spices.
2) Once the liquid is boiling and steaming add tomatoes and onion. Gently stir to incorporate.
3) Reduce heat and simmer for 20 to 25 minutes. Shut off heat and allow to cool to room temperature. Place in a serving bowl and serve.