Heat the milk in a wide-based, shallow saucepan to scalding point (it should barely shiver). Add the haddock, in a single layer, and poach gently for 6–8 minutes until the flesh flakes easily. Lift the fish from the pan with a slotted spoon. Remove the skin and break the flesh into large flakes. Strain the milk into a jug.
Cook the pasta in a large saucepan of salted boiling water according to the packet instructions until al dente.
Meanwhile, melt the butter in a saucepan over a very low heat. Add the flour and cook, stirring, for 2 minutes until it is a light biscuity colour. Remove from the heat and slowly add the reserved milk, stirring away lumps. Return to the heat and simmer, stirring, for 2–3 minutes until creamy. Stir in the mustard, cream, peas, Cheddar and half the Parmesan. Season with salt and pepper.
Drain the pasta and return to the pan. Fold the sauce and haddock flakes into the pasta and transfer to a greased ovenproof dish. Mix the parsley and remaining Parmesan into the breadcrumbs and scatter evenly over the pasta. Drizzle with the oil and bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 10 minutes until bubbling and golden.