This super-quick fish pie can be made in a heartbeat and you can change up the fish to vary the taste.
Preheat the oven to 200°C (fan 180°C), 400°F, Gas Mark 6.
Bring a medium pan of salted water to the boil and cook the potatoes together for 10–12 minutes or until tender. Alternatively, steam the potatoes, in which case they tend to take on less water, making for a stiffer, drier (and better) sweet potato mash.
As this cooks, put the cornflour in a wide pan with a little of the milk and stir until dissolved and smooth. Then add the rest of the milk while stirring all the time. Place on a low to medium heat and while continuing to stir all the time, bring to a simmer and cook for a few minutes until slightly thickened. Then add the fish, peas, spring onion and dill and cook gently for about 3 minutes, stirring regularly to prevent it from catching on the bottom. Carefully stir in the crème fraîche and salt and pepper, to taste. Remove from the heat and tip into a 2.5 litre ovenproof dish (mine measures 25cm square and 6cm deep).
Once cooked, drain the potatoes well and tip them back into the pan. Mash until smooth (or use a Mouli grater or potato ricer) with the butter, nutmeg and enough salt and pepper to taste. Place spoonfuls of the mashed potato all over the top of the fish filling and spread it out evenly with a fork. Grate over a little more nutmeg if you fancy it and pop on a baking tray in the oven for about 20 minutes until the top is just catching colour and the fish filling is piping hot. Serve at once.
Lorraine will be appearing at the BBC Good Food Show Winter (NEC Birmingham 26-29th November 2015).