Cut the sweet potato into chunks and steam or boil in salted water until soft. Peel and mash 1 lb of the potatoes with 1 egg, 1 oz butter and enough milk to give it a soft consistency.
Hard boil the remaining eggs, then cool, shell and quarter.
Put 1/2 pint milk into a pan with the sliced onions, peppercorns and dill stalks. Bring gently to the boil.
Lay the fish fillets in an ovenproof dish and pour over the infused milk with its seasonings. Bake at 180 c Gas 4 for 15-20 minutes until cooked. Strain off the milk and set aside, discarding the onions and seasonings.
Flake the fish and lay on the base of a pie dish. Cover with peas and then the quartered eggs. Sprinkle with the chopped dill leaves.
Make a white sauce with 1/2 oz butter, the flour and the reserved milk. Simmer for 5 minutes and season generously. Pour over the fish, shaking to distribute.
Spread the mashed potato thickly over the top and make a wavy pattern with a fork. Dot with the remaining 1/2 oz butter.
Bake at 230 C Gas 8 for 10-15 minutes until browned.