Coat both sides of fish with peanut oil. Put lemon grass, ginger, and lime slices inside body of fish. Pour about 1 inch of water in a large shallow roasting pan. Put a steaming rack in pan and put pan on two burners on the stove and let water boil.
Lay fish on rack, place lid on pan, lower heat, and simmer 18 minutes or until fish is flaky. Move fish to a serving platter.
Mix together mint, lime juice, shallot, chile, fish sauce, and garlic and spoon over fish. Top with mint sprigs.