How to Make It
In a small bowl, combine the salt with the cumin, peppercorns, mustard seeds, sugar, turmeric and cayenne.
In a large skillet, warm 3 tablespoons of the vegetable oil over moderately high heat. Add 6 of the curry leaves and when they begin to crackle, add half of the onions. Reduce the heat to moderately low and cook, stirring, until the onions are softened but not browned, about 4 minutes. Add half of the garlic and cook, stirring, until fragrant, about 2 minutes. Add half of the spice mixture and cook, stirring, for 1 minute.
Push the onions to one side and add half of the tomato slices to the skillet in a single layer. Cook over moderately high heat until the tomatoes are browned on the bottom, about 1 minute. Turn each slice of tomato and brown the second side. Stir lightly to blend with the onions. Slide the tomatoes onto a platter and cover with foil to keep warm. Wipe out the skillet and repeat with all of the remaining ingredients. Serve hot.