2.5 kg tomatoes (blanched, peeled and grossly chopped)
1 head of celery
6 onions, chopped (I like to mix white and red onions)
3 granny smith apples, peeled and chopped
1/4 cup salt
Mix everything together and let sit for 2 hours, then drain to remove as much liquid as possible. Once that's done, add:
1 tsp mustard seeds
1 tsp ground black pepper
Mix together 1 cup and 1 cup white vinegar in a bowl. Add just enough to fully cover the veggies. If you need some more, mix up another batch of sugar/vinegar in a 1:1 ratio. Once the veggies are just covered, add another 3/4 cup sugar.
Bring to a simmer and cook for 3 hours, mixing regularly. Adjust sweetness/tartness to taste while it's cooking. Once it's done, put in sterilized pots.
To sterilize pots (I use jam jars, but mason jars work just as well), wash in warm soapy water, rinse with very hot water, place upside down on a baking tin and leave in a medium oven for 15 minutes. Boil lids. Fill while warm and cover promptly. Tighten lid and Bob's your uncle.
PS: For a bit more of a kick, you can add one or two finely chopped chilies.