Chicken Casserole with Mustard And Tarragon

¼ C olive oil
1 kg chicken pieces
1 onion, finely chopped
1 leek, sliced
1 clove garlic
350g mushrooms, sliced
½ t tarragon
1 ½ C chicken stock
¾ C cream
2 t lemon juice
2 t Dijon mustard
Salt and pepper
Preheat oven to 180°C. Cook chicken in batches in 1 T oil until golden. Add remaining oil and cook onion, leek and garlic 5 mins. Add mushrooms 5-7 mins. Add tarragon, stock, cream, lemon, mustard, bring to boil 2 mins. Return chicken to pan and season well. Pour into ovenproof dish and cook 1 hour, or until sauce reduced and thickened. Serve with potatoes and veggies/salad.