In a pan cook onions slowly on low to moderate heat until translucent. Add garlic and chili to the onions. Stir in the mustard seeds, turmeric and cumin seeds and continue cooking. Add ginger, cook briefly, then add crushed tomatoes water and a few twists of pepper. Bring to a boil, then add the whole tomatoes.
Turn the heat douwn and simmer covered with a lid about 25 to 35 Minutes, turning the tomatoes once or twice during cooking. They should be softened, but not totally collapsed.
Push the tomatoes to one side and stir in the yoghurt and let the sauce heat through. Season with salt and pepper to taste. Serve over rice or with naan.