Heat 2 tablespoons of oil in a large, hot frying pan and brown the chicken on both sides. Remove from the pan and set aside.
Add the chopped leeks and garlic to the pan and fry for 30 seconds - 1 minute until soft and fragrant.
Mix together the cream, chicken stock and mustard. Pour into the pan.
Allow to simmer for 2 minutes then add the browned chicken thighs back into the pan. Turn down the heat and allow to simmer for 20-25 minutes, covered, until the chicken is cooked through. If needed, add more stock to the pan.
When the chicken is cooked, season with lemon juice, salt and pepper.
Sprinkle over the chopped parsley then serve with rice and steamed greens.
I often adjust the ratio of liquids to create more sauce, especially when serving this dish with rice. I use 2 cups cream to 1½ cups stock and will often add more stock during the cooking time.