Peel the skin off the smoked mackerel and discard. Put the fish in a food processor, breaking it up slightly as you go. Add the cream cheese, the zest and most of the juice of 1 lemon and a few leaves of parsley. Whiz for 20 seconds or so, or until you get a nice creamy pâté. Season to taste.
Toss the snipped cress, remaining parsley leaves and celery leaves together in a large bowl, and add the celery sticks. Dress with a good squeeze of lemon juice, a splash of extra virgin olive oil and some salt and pepper.
Heat a griddle pan. When hot, add the bread, in batches, and press down with something heavy like a frying pan so the bread is squashed against the griddle ridges. Toast for about 1 minute, turning halfway.
Cut the remaining lemon into wedges and serve alongside the pâté with the griddled toast, the herb salad and a nice glass of chilled white.