In a large frying pan over medium heat, melt butter and sauté rosemary and garlic until tender. Stir in mushrooms and sauté until juices run. Add parsley and stir occasionally to prevent sticking. (Add more butter if needed.)
When mushrooms are tender, stir in stock and mix well before adding cornflour. Cook for 1 to 2 minutes, then add whiskey and cook for 1 minute more.