In large skillet saute the onion till translucent. Add the ginger and garlic and cook one minute. Add curry powder and stir thoroughly. Pour in coconut milk and tomatoes and stir to combine. Cook on medium to a simmer. Add shrimp and cook on low until just starting to turn pink. Add cilantro and lemon juice. Season to taste with salt and pepper. Pour into serving dish immediately to stop the cooking. Serve with basmati rice.