Dry the prawns and scallops on kitchen paper and season.
Finely chop the shallot and finely grate the Parmesan.
Heat butter in a pan over a medium heat and add the scallops and prawns. Cook for about 3 minutes or until the shellfish is opaque. Transfer to a bowl and keep warm.
Add the chopped shallots to the remaining butter in the pan and cook for about 3 minutes until softened but not coloured.
Add the wine and stock to the pan and let it bubble for a few minutes until the liquid has reduced by half.
Stir in the cream, half of the chopped parsley and mustard and heat through for about 2 minutes until slightly reduced. Check the seasoning.
Drain any liquid from the shellfish and divide between four serving dishes.
Pour the sauce over the top.
Mix the panko with the Parmesan and sprinkle over the top.
Place under a medium to high grill (broiler) for about 2 minutes until bubbling and golden.
Scatter over the remaining parsley before serving.