Preheat the oven to 220C/Gas7; cook the potatoes in boiling water for 15 minutes, until tender, drain and set aside
Put the leeks into a frying pan with 100ml water, bring to the boil, cover and cook for 5 minutes; transfer to a plate; put the fish into the frying pan with the milk and bay leaf, cover and slowly bring to the boil; remove from the heat; transfer the fish to a plate; strain the milk into a jug and discard the bay leaf; flake the fish into an ovenproof dish
Melt the butter in a pan, stir in the flour to make a paste; cook stirring for 2 minutes; slowly whisk in the milk; simmer for 5 minutes stirring until smooth; add the capers, parsley and lemon zest and season
Defrost the spinach and squeeze out the excess water; chop and scatter over the fish; pour over the sauce; crush the potatoes with the oil and leeks, season and spread over the top
Bake for 20 minutes until golden and serve
6 servings