Cook the onion and garlic in the butter in a heavy 5 quart pot over moderate heat, stirring, until the onion is softened, about 4 minutes. Add the spices and flour and cook, stirring constantly, for 1 minute. Whisk in the cream, coconut milk, broth and lime zest and bring just to a boil. Simmer the mixture, stirring constantly, until it begins to thicken, about 2 minutes. Add the shrimp and simmer, stirring, until the shrimp turns pink and is cooked through, about 4 minutes. Stire in the lime juice, and add salt and pepper to taste.
Ladle the curried shrimp over rice and top with garnishes as desired. Serve the mango chutney on the side.