Loosen mackerel fillets from the skin, remove any bones. Tear fish flesh finely with two forks. Wash the lemon hot, dry it and rub the skin thinly. Halve the lemon, squeeze one half. Wash parsley, shake dry and chop finely.
Mix the fish, lemon zest, parsley and cream cheese with a fork. Season to taste with salt, pepper and lemon juice. Spread on the slices of grey bread and serve.