Pre-heat the oven to 400°F or Gass Mark 6.
Start by prepping the chicken. In a bowl mix together the butter, garlic, tarragon, mustard and season with salt and pepper. Using clean hands, smear this mixture over the entire chicken and underneath the skin. Place the chicken on to a roasting tray and into the preheated over for 1.5hrs.
Place a large saucepan over a medium heat and add 1.5oz of butter, shallots and garlic, cover and lower the heat to allow the shallots to soften (but not brown), this will take about 2 minutes. Then stir in the rice and coat it with the butter and stir for a couple of minutes. Then add the wine and stir and until absorbed.
Slowly pour in the hot broth, ladle by ladle until all the broth is absorbed stirring all the time. It should take between 15 to 20 minutes for the rice to cook. But taste as you go.
While the rice is cooking, place a frying pan over a high heat and add the remaining butter and the sliced chestnut mushrooms. Cook for 3 minutes.
About 5 minutes before the rice is cooked stir in the mushrooms and fresh thyme followed by the mascarpone and parmesan. Season with sea salt and freshly ground black pepper.
Once the chicken is cooked, place on a large serving platter and spoon the risotto all around it. Garnish with fresh thyme.