Heat the vegetable oil in a large skillet. Add the chopped onion and cook over moderately high heat, stirring frequently, until the onion is golden, about 5 minutes. Add the minced garlic and chiles and cook until softened, about 2 minutes. Stir in the ground cumin, coriander, garam masala and turmeric. Cook the ground spices for 1 minute, stirring constantly. Reduce the heat to moderate, add the chopped tomatoes and cook, stirring occasionally, until the sauce is thickened slightly, about 5 minutes. Stir in the lemon juice, brown sugar, cilantro, salt and water. Bring to a boil. Add the shrimp and cook, stirring frequently, until they're pink and curled, about 5 minutes. Transfer the shrimp and sauce to a shallow bowl and serve at once.