Boil the potato slices for 10 mins then drain, tip into a bowl and gently mix in the milk and half the cheese. Don’t worry if the potatoes break up a little.
Meanwhile, heat the oil in a large frying pan and cook the onion until softened. Stir in the pepper and cook for 5 mins more. Spoon in the balsamic vinegar and bouillon powder, then stir in the tomatoes, basil and garlic. Lay the cod fillets on top, then cover and cook for 6-8 mins until the cod flakes when tested. Heat the grill to high.
Take off the heat, stir in the prawns and tip into a shallow baking dish, breaking up the cod into large chunks. Cover with the potatoes and sprinkle with the remaining cheese. Grill until golden. While it’s grilling, steam or boil the broccoli to serve with the bake.