Par-boil potatoes in salted water for 7-10 minutes, then drain and reserve.
Fry chicken thighs in splash of oil and a good pinch of salt and pepper over a medium high heat for 4-5 minutes each side. Once cooked, remove chicken thighs from frying pan, place on plate and set aside to rest.
Fry the onions and garlic in the pan, add butter and season. Cook for 3 minutes until onions soften then add mushrooms and chopped tarragon and cook for another couple of minutes, stirring frequently.
Reduce heat to low and add soup, potatoes, mustard, cream, chicken thighs and any juice from resting. Shake pan to keep contents moving a little for another minute just to warm the sauce then pour everything into a baking dish (or leave in the pan if oven proof) and bake in oven for 25 minutes.
Five minutes before the chicken is done, gently boil the cabbage in salted water for a few minutes, then drain and serve alongside the chicken.