Preheat the oven to gas mark 6.
Heat the oil in a large, heavy non stick pan over a high heat.
Add the chicken to the pan for 2 minutes and cook each side, then add to an oven/casserole dish.
Pop into the oven for 20 minutes.
Meanwhile, add the butter into the pan and allow it to brown for a minute over a low heat.
Add the shallots, garlic and green chilli. Braise for 10 minutes and remove from the heat.
Once the chicken is done, add the chicken and stock to the pan and cook over a high heat until the stock reduces. (About 15 - 20 minutes).
Add the cream and mushrooms, mustard, tarragon, seasoning and reduce the heat to low. Cook for 5 minutes.