Puree cucumber, dill, onions, salt and yogurt in blender. Pour into small bowl; set aside. Arrange fish on roasting rack. Sprinkle with lemon juice. Cover with wax paper. Microwave at High 5 to 7 minutes or until fish flakes easily, rearranging after half the cooking time; set aside. Reduce power to Medium. Microwave cucumber dill sauce 1 to 3 minutes to heat. Serve over fish.