Season the chicken with salt, black pepper and nutmeg (if using), and leave for at least an hour in the fridge
Cook the onions in 1tbsp oil in a large sauce pan a for 10-15 minutes soft and golden and sweet.
Meanwhile, heat a frying pan until very hot, then add 2tbsp oil and the chicken and cook for about 5-7 minutes until beginning to brown and is just cooked through, then tip into the pan with the onion
Add the remaining oil to the frying pan and cook the mushrooms for 3-5 minutes until softened. Tip into the saucepan with the onion and chicken
Put the stock and mustard in the frying pan with the mushrooms and bring to the boil, stirring well. Add the sour cream and season to taste. Stir into the chicken in the saucepan and simmer gently for 5-10 minutes.