Heat the oven to 190°C/ 170°C fan/gas 5. Heat the oil in a medium cast-iron or ovenproof frying pan over a medium heat. Cook the fennel and leek, stirring, for five to seven mins or until softened. Add the dill, chives and half the parsley.
Whisk the eggs and zest in a large jug and season with freshly ground black pepper.
Combine the egg and leek mixtures and cook for two to three mins or until beginning to set around the edges. Transfer to the oven and bake for 15 min or until golden and set.
Top the frittata with the salmon. Scatter over the capers, remaining parsley, dill and onion, if using. Slice and serve with the salad leaves.
Pickle an onion: Thinly slice a red onion and put in a bowl with two tbsp white wine vinegar and one tsp sugar. Mix well. Leave 15 min, then drain.