Preheat the oven to 180°C/gas mark 4.
Boil the sweet potatoes and butternut squash for 10 minutes, or until soft. Mash together with half of the butter and set aside.
Heat the butter in a frying pan over a medium–high heat. Cook the spring onions and carrots for about 5 minutes until soft, then add the cream, mustard, lemon juice and chives, and gradually add half the cheese until it melts.
Place the fish in a baking tray, and sprinkle the hard-boiled eggs all over. Pour the cheese sauce on top and add the peas. Layer the potato and butternut squash mash all over the fish and scatter the remaining cheese over the top.
Cook for 25 minutes. Serve with extra peas.