Preheat the oven's grill. Cover a medium baking dish with foil.
Melt butter with cream in a saucepan over medium heat then blend in the mustard. Cook and stir until thickened. Mix in crab and prawns. Season with Cajun seasoning, salt and pepper. Cook until heated through.
Place fish in the prepared baking dish then rub with olive oil, lemon juice and parsley. Season with salt and pepper.
Grill fish for about 4 minutes on each side until easily flaked with a fork. Spoon the crab and prawns mixture over fish to serve.