In a medium skillet over medium-low heat, warm the oil.
Add the cumin, coriander, turmeric, mustard seeds and chili powder and cook, stirring often, until fragrant, 2 to 3 minutes.
Add the garlic, Thai chile and 2 tablespoons of the vinegar. Cook until the vinegar has evaporated and the pan is dry, 4 to 5 minutes.
Add the tomatoes, brown sugar, salt and remaining ½ cup of vinegar and cook, stirring often with a rubber spatula, until thickened and dark in color, 40 to 50 minutes.