Put 3 tablespoons of oil in a large frying pan (for which there is a lid), about 10 ¼ inches/26 cm wide, and place over medium heat. Add the mustard seeds and 10 curry leaves and cook for 1 minute, until the seeds start to pop. Add the onion and cook for about 7 minutes, stirring frequently, until they begin to color. Add the ginger, garlic, coriander stems and fresh tomatoes and cook for 5 minutes, then add the fennel spice mixture. Cook for 2 minutes, until fragrant, then add the sambal oelek, canned tomatoes, tamarind, water, sugar and 1 ¼ teaspoons salt. Stir well and bring to a simmer. Reduce heat to medium-low and cook for 20 to 25 minutes, uncovered, until thickened.
