Make the sauce: In a large heavy saucepan cook the onions in the oil over moderate heat, stirring, until they are softened, add the gingerroot and the garlic, and cook the mixture, stirring, for 1 minute. Add the bell peppers and cook the mixture, stirring, until the peppers are softened. Add the curry powder and the flour and cook the mixture over moderately low heat, stirring, for 3 minutes. Add the broth, bring the mixture to a boil, whisking, and stir in the tomatoes. Simmer the sauce, stirring occasionally, for 5 minutes and stir in the lime juice and salt and pepper to taste. The sauce may be made 3 days in advance and kept covered and chilled. Reheat the sauce before continuing with the recipe.
Arrange the snapper on a deep ovenproof platter, ladle about 2 cups of the sauce over it, keeping the remaining sauce covered and warm, and cover the platter tightly with oiled foil, oiled side down. Bake the snapper in the middle of a preheated 350 degree F. oven for 45 to 55 minutes, or until it just flakes. Garnish the snapper with the coriander sprigs and serve the remaining sauce separately.