Pour the milk into a shallow pan, reserving 1 tablespoon. Cut the fish into pieces, checking carefully for bones, and add to the pan. Bring to a simmer and cook for 3-5 minutes until opaque and cooked through. Remove the fish using a slotted spoon, drain on kitchen paper and check again carefully for any small bones. Strain the cooking milk into a jug and set aside.
For the mash, cook the swede and carrots in boiling water until tender. Drain and mash with the reserved milk, then season with black pepper.
Preheat the oven to 200°C, fan 180°C, gas 6. Melt the butter in a small pan over a medium heat. Add the flour and stir until the mixture comes together. Remove from the heat, add a little of the cooking milk and stir. Return to the heat and slowly add the rest of the milk, whisking constantly, until smooth. Remove and stir in the parsley. Season with black pepper
Heat the oil in a frying pan and cook the leeks for 3-4 minutes until softened. Put the leeks, fish and peas in a 1-litre ovenproof pie dish. Pour over the sauce and gently toss together. Top with the mash, fluffing up with a fork. Cook in the oven for 35 minutes. Cool slightly before serving.