Wipe the mushrooms with a damp cloth and finely slice. Melt 2tbsp butter and 2 tbsp olive oil in a heavy non-stick frying pan and gently cook the onion for 10 minutes, until soft and translucent.
Add mushrooms and cook for 5 minutes. Add sea salt, pepper, paprika and wine and bring to the boil. Reduce the heat and cook for 5 minutes until the wine is absorbed. Add the tomato paste and stock and simmer, stirring, for 5 minutes.
Lightly bash the chicken thighs flat and cut across the grain into thin strips. Heat the remaining butter and oil in a second pan and cook the chicken in batches over medium heat until lightly golden.
Tip the chicken and any pan juices into the mushroom suace, add the sour cream, mustard and dill, and simmer gently, stirring, for 5 minutes. Scatter with parsley and serve with rice or buttered noodles.