Preheat the oven to 180oC/350oF/Gas Mark 4.
Heat 1 tbsp of olive oil in a heavy bottomed casserole pan over a medium heat and cook the chicken thigh fillets in two batches for 6 – 7 minutes. Remove from the pan.
Add the remaining oil to the pan and cook the onion, leek and garlic over a medium heat for 5 minutes. Add the mushrooms and cook for 5 – 7 minutes or until they are soft and browned and most of the liquid has evaporated.
Add the tarragon, chicken stock, cream, lemon juice and mustard, bring to the boil and cook for 2 minutes. Return the chicken pieces to the pan and season. Cover.
Place the casserole pan in the oven and cook for 1 hour, until the sauce had reduced and thickened.
Season to taste with salt and pepper, and serve with potatoes and a green salad.