Peel the skin off the mackerel and discard, separate the meat from the bone removing any unwanted bones. Flake three quarters of the mackerel into a food processor, breaking it slightly as you go. Add the red onion, cream cheese, crème fraiche, lemon zest, lemon juice and horseradish. Whizz until you reach the consistency you like approximately 20-30 seconds or until you get a nice creamy or course pâté. Season to taste.