Peel the potatoes, cut them into pieces and boil them in a pot (with a pinch of salt) for approximately 10-20 minutes, checking with a fork to see if they are fully cooked.
In the meanwhile, prepare the other ingredients. Season the chicken with pepper and salt.
Melt 20g of the butter in a flameproof casserole dish or saute pan over a medium heat. Add the chicken and pancetta to the dish and fry for 5 mins until the pancetta is crispy and the chicken browned and cooked through. Set aside in a bowl.
Don't forget to check the potatoes... drain them once they are cooked. Set aside or continue to make the mash if you have the time. Set 20g of the butter aside for the mash.
In parallel: Lower the heat on the casserole dish or saute pan, add the remaining butter (40g) and, when it starts foaming, add the onions. Cook for 10 mins until softened, then stir through the flour and cook for another 2 mins. Remove from the heat and slowly whisk in the chicken stock. Return to the hob, bring to the boil and cook for 5 mins. Stir through the tarragon, mustard and cream, followed by the chicken and pancetta, and cook for another 5 mins until the sauce is thickened.
Make the mashed potatoes: add the nob of butter, some freshly ground pepper and mash. Add milk until you reach the desired consistency.
Heat the grill to high. If you used a saute pan for the chicken, add it into a flameproof casserole dish. Spoon the mashed potatoes on top of the chicken filling. Smooth and swirl the potato over right to the edges, drizzle with a little olive oil and put under the grill for about 10 mins or until the top is golden and bubbling.