1. In a large skillet, heat 2 tablespoons of olive oil. Add the mustard seeds and cook over high heat, stirring constantly, until they begin to crackle, about 1 minute. Add the onion, reduce the heat to medium and cook, stirring frequently, until the onion has softened slightly, about 2 to 3 minutes. Add the garlic and red-pepper flakes and cook, stirring frequently, for 5 minutes longer.

2. Add the cumin, turmeric, garam masala, clove, cinnamon, black pepper, salt, sugar, vinegar and tomato sauce. Bring to a boil, then reduce heat to low and simmer, uncovered, for 20 minutes.

3. Remove the pan from the heat and, when the mixture is cool enough, puree in a blender or food processor until smooth; set aside.

4. In a large, heavy skillet, heat the remaining 2 tablespoons of olive oil. Add the shrimp and curry leaves and sauté, stirring and shaking the pan, until the shrimp are no longer transparent. Add the reserved sauce, reduce the heat to low and cook until the shrimp is just cooked through, about 3 to 5 minutes. Serve immediately.