Put the milk, bay leaf and onion in a wide pan and bring to a gentle simmer. Add the white fish and smoked haddock, poach for 5 minutes until just opaque, then lift out with a slotted spoon. Strain and keep the milk.
Melt the butter, stir in the flour and cook for 1-2 minutes. Gradually whisk in the poaching milk until smooth and thick. Simmer 3-4 minutes, then stir in the cheese, parsley, lemon juice, salt and pepper.
Boil and mash the potatoes with butter and milk. Flake the fish into a baking dish, add prawns, peas and eggs if using, pour over the sauce and fold gently. Top with the mash, spreading to the edges.
Dot with extra butter and bake at 200°C (180°C fan) for 25-30 minutes until golden and bubbling.