In a medium sauce pan, cook chopped tomatoes over medium heat until it reduces down. Stir occasionally as it will start to thicken and start sticking to the pan. Depending on the tomatoes, it can take anywhere from 45 minutes - 1 hour. Also, keep an eye and adjust the heat level to low-medium if needed.
Add ginger-garlic paste, turmeric powder, red chili powder, and salt to the pan and cook it further to almost like a paste consistency. You should see no more liquid in the pan. This takes another 30-45 minutes over low-medium heat. Turn off the heat.
In another small pan, heat oil and fry garlic cloves and dried red chili until they turn dark brown but not burnt. Add cumin seeds, mustard seeds to the pan and curry leaves until mustard seeds start to pop and does a little dance.
Pour the hot oil mixture over the tomato paste and stir everything. Adjust salt per taste.
Transfer the tomato achar into a clear jar with a lid. Keep the jar outside in a cool area or store in the fridge and enjoy.