Heat your oven to 200°C/Fan 180°C/Gas 6. Crumble the stock cube into a measuring jug and stir in 600 ml boiling water. Add the dried porcini mushrooms and set aside to infuse.
Peel the beetroot and finely slice into matchsticks. Put in a bowl with a large pinch of salt. Scrunch together. Pour in the brown rice vinegar. Stir and set aside to pickle.
Scrub the potatoes and chop into 1 cm cubes. Chuck in a large pan and cover with boiling water. Add a pinch of salt. Bring to the boil and simmer for 5 mins. Drain and leave to dry.
Heat ½ tbsp oil in a heavy based pan over a medium heat. Tip in the diced chicken leg. Fry for 5-6 mins, stirring occasionally, till golden on all sides. Pour in the porcini and porcini infused stock. Bring to the boil, then turn down the heat. Cover and simmer for 30 mins.
Tip the potatoes onto a baking tray lined with baking paper. Drizzle over 1 tbsp oil and season. Roast for 20 mins till crisp and golden. Turn the potatoes after 10 mins.
Thickly slice the mushrooms. Peel and grate the garlic. Peel and thinly slice the onion. Heat ½ tbsp oil in a frying pan. Fry the mushrooms for 2 mins, then throw in the onion and garlic. Cook for 4 mins till all the veg is just softened. Add the veg to the chicken pan and simmer for a further 10 mins.
Finely chop the chervil stalks and put in a bowl. Tip in half the pot of soured cream. Ladle a couple of spoonfuls of the stock from the chicken pan into the soured cream. Mix, then pour int0 the chicken pan. Cook for 2 mins till warmed through. Season.
Roughly chop the chervil leaves. Add half to the chicken and the remainder to the pickled beetroot. Serve with the roasted potatoes.