Place the trout fillets on a board, skin-side up. Lightly season both sides of the fillets and place in a medium non-stick frying pan on a medium-high heat. Cook for 3 minutes on one side, then turn and cook for another 3 minutes. (The skin should be beautifully crisp and the flesh blushing in the middle.) Remove to a plate to rest.
Meanwhile, place a large handful of the watercress in a blender with the juice of half the lemon and 1 teaspoon of extra virgin olive oil. Blitz until smooth, then fold in the yoghurt. Season with black pepper and tip into a small bowl.
Finely slice the cucumber, spring onion and avocado, then dress with a little lemon juice and some black pepper.
Lightly toast the bread and spread each slice with the watercress yoghurt. Top each with some avocado, cucumber and spring onion slices. Break the crispy trout skin into shards and the fish into chunks. Divide the trout pieces between the toasts and top with the crispy skin.
Snip over some cress and serve with the remaining watercress on the side and a handful of your favourite crisps, if you like.