Preheat the oven to 200°C/fan oven 180°C/gas mark 6.
Cut each chicken breast into thin strips, place in a dish, and pour over the lemon juice and sprinkle over the zest and thyme. Leave to marinate for 30 minutes.
Heat a large non-stick frying pan, add the rapeseed oil and fry the chicken strips for about 5-6 minutes on either side until they are thoroughly cooked all the way through. Remove and cool.
Now, melt the butter in the frying pan and add the leeks and mushrooms. Cook gently for 10 minutes or until the leeks are tender. Add the creme fraiche and cook for 1 minute. Season with some black pepper. Transfer to a plate and spread the mixture out to allow it to cool quickly.
Drain the anchovies, roughly chop them and place them in a small bowl. Add the mustard and mix well. Season with a little black pepper.
Dust a work surface with flour, roll out two-thirds of the pastry and use it to line a 25cm springform cake tin, leaving about a 2.5cm overhang. Spread the anchovy and mustard mixture all over the bottom of the pastry.
Put half the chicken fillets on top of the anchovy mixture. Cover with half the leek and mushroom mixture. Spoon the other half of the chicken on top and finish with a final mushroom layer.
Roll out the remaining pastry to fit on the top of the pie. Lay it over the top and then bring the overhang of pastry into the centre to meet it. Crimp and fold the edges over to make it all snug. Brush the pastry with beaten egg. Transfer onto a baking tray and chill for an hour or so or overnight if you want.
Bake the pie for 35-40 minutes or until the pastry is golden brown and the inside is piping hot. Use a long knife to test how hot it is - gently pop it into the middle of the pie, leave it in for 15 seconds, pull it out and if the knife is hot, so is the filling! It may take a little longer to cook, to ensure the filling is hot, if the pie has been in the fridge for a while.
If this is the case, cover it with foil after the 35-40 minutes and continue to cook until the filling is piping hot.
Slice and serve the pie with steamed broccoli, peas and carrots.
Recipes taken from 'Lotte's Country Kitchen' by Lotte Duncan, priced £20, published by Absolute Press. Photography © Lara Holmes.