Heat half the oil in a large pan and brown seasoned chicken pieces until golden.
Remove chicken and fry pancetta until crisp, then sauté shallots, garlic, and mushrooms for 3 minutes.
Return chicken to the pan and cook for 2 minutes.
Add butter, Madeira, stock, 300ml cream, and tarragon, then simmer for 20 minutes.
Boil potatoes in salted water for 20 minutes until soft.
Drain potatoes, mash thoroughly, and mix with milk, 300ml cream, and 150g butter.
Blanch green beans in boiling water and refresh in ice water.
Sauté beans in a small pan with butter until tender and coated.
Serve the chicken and mushroom sauce alongside the mash and green beans.