Peel and chop the potatoes into bite-size pieces. Pop them in a large pan and cover with cold water. Clamp on a lid, bring up to the boil then turn the heat down a little and simmer for 15 mins till tender when pierced with a knife.

Meanwhile, trim the root and top 2cm off the leek, then slice a cross in the top to open it out. Rinse the leek under a cold tap and finely slice it. Peel the onion and finely chop it. Peel the garlic and finely chop it. Pull the leaves and silks off the sweetcorn. Cut a small slice off the cob's base so it will stand upright on your board then run your knife down the sides slice off the corn kernels.

Melt half the butter in a large pan on a medium heat. Add the onion, leek, garlic and corn kernels. Season with a pinch of salt and pepper and fry for 10 mins, stirring occasionally, till everything is tender and lightly browned.

When the potatoes have cooked, drain them and leave to steam dry in the colander for 1 min. Tip the potatoes back into the pan and add the remaining butter and a pinch of salt and pepper. Mash using a masher or fork till smooth. Cover the pan and set aside to keep the mash warm.

Pour the cream into the pan of browned veg and add 100ml cold water. Add 1 tbsp capers. Roughly chop the tarragon leaves and add them to the pan. Cook till just starting to simmer, then add the fish pie mix and stir. Cook for 5-6 mins, turning the fish pieces once or twice, till tender.

Squeeze a little lemon juice into the pan and taste. Add more salt, pepper or lemon juice if you think the dish needs it.

Divide the mash between 2 warm plates. Spoon over the creamy fish and veg. Serve with wedges of the remaining lemon on the side.