Welsh rarebit topped fish pie

Method

1. Pour 450ml of the milk and the whipping cream into a large pan
   and bring to the boil. Add the fish and prawns, then simmer for 2-3 minutes until just tender. Remove with a slotted spoon (reserve the liquid) and put in the baking dish. Keep warm.
2. Melt 50g of the butter in a pan, add the flour and cook, stirring, for 2 minutes. Remove the pan from the heat and gradually whisk in the reserved fish cooking liquid. Return to the heat and slowly bring to the boil, whisking all the time. Simmer for 5 minutes, remove from the heat and allow to cool for 10 minutes, then season and stir in the peas.
3. Meanwhile, put all the ingredients for the rarebit sauce, apart from the egg yolks, into a small pan and heat gently, whisking until thick. Take off the heat, then whisk in the yolks and season to taste (see Make Ahead).
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4. Cut the potatoes into large chunks, then boil in salted water for 20 minutes or until tender. Heat the grill to medium.
5. Drain the potatoes well, return to the pan to steam dry, then mash until smooth. Add the remaining butter, milk and half the rarebit sauce, mix until smooth (see Make Ahead), then spoon over the fish pie filling. Drizzle the rest of the rarebit sauce on top. Grill for 10–15 minutes until bubbling and golden brown. Stand for 10 minutes, then serve.