In a large saucepan, heat the oil until shimmering. Add the onion, garlic, jalapeño and ginger; cook over moderately high heat, stirring, until softened, 5 minutes. Add the curry powder and cook until fragrant, 1 minute. Add the tomatoes and their juices, the coconut milk and sugar; bring to a boil. Season with salt and pepper and simmer over moderate heat, stirring occasionally, until slightly thickened, 20 minutes. Stir in the cilantro and transfer to a bowl.
The sauce can be refrigerated for up to 5 days.
Use as a poaching liquid for chicken, shrimp, lamb, pork and vegetables. Add to a sautéat the last minute. Toss with boiled potatoes or serve over rice. Mix with chicken stock and vegetables or seafood for a quick soup.