Preheat the slow cooker if necessary; see the manufacturer's instructions. Lightly oil a 1 litre (1¾ pint) soufflé dish and base-line with a circle of nonstick baking paper, checking first that the dish will fit in the slow cooker pot. Blend the haddock or cod loin, egg whites, lemon rind, half the lemon juice and salt and pepper in a food processor until roughly chopped, then gradually add the cream and blend until just beginning to thicken.
Arrange half the smoked fish slices over the base of the dish. Spoon in half the fish mousse and spread it level. Mix the fish fillet with a little remaining lemon juice and some pepper, then arrange on top. Top with the remaining fish mousse, then the smoked fish slices.
Cover the top with foil and lower into the slow cooker pot. Pour boiling water into the pot to come halfway up the sides of the dish. Cover with the lid and cook on high for 3–4 hours or until the fish is cooked through and the terrine is set.
Lift the dish out of the slow cooker pot using a tea towel and leave to cool for 2 hours. Loosen the edge, turn out on to a plate and peel off the lining paper. Cut into thick slices and serve with salad and toast.