Remove the skin from the mackerel and flake the flesh into a bowl. Add the crème fraîche, lemon juice, dill and parsley, and gently mix until well combined. Season to taste, then fold in the apple matchsticks, if using.
Toast the crumpets in a toaster or heat your air fryer to 180C for 2 mins before adding your crumpets for 3-4 mins. Remove and generously butter them. Spread the mackerel mixture over the crumpets. Garnish with the sliced cucumber, radishes, and a sprinkle of chives, if you like.
