Heat the oil in a very large skillet over medium heat. Add the chicken and brown on all sides; transfer to a casserole. Add the mushrooms and garlic to the skillet and cook until the mushrooms are tender, about 5 minutes. Mix in the flour until well combined. Slowly mix in the wine, and then the stock. Stir in the sage, salt and pepper and bring to a simmer. Stir in the whipping cream, then pour over the chicken and gently stir to combine. If serving now, cover and cook in a 350 degrees F for 30 minutes. If bringing to a party, cool to room temperature, cover and store in the fridge and finish in the oven when needed. Cooking time will be 5 to 10 minutes longer as you will be starting from cold. Sprinkle with parsley just before serving.