In a 5-quart Dutch oven or saucepan, heat 2 tablespoons oil over medium-high heat. Add leeks and garlic, stirring frequently until tender. Add remaining tablespoon of oil along with mushrooms.. Cook, stirring frequently, until mushrooms are tender, about 10 minutes.. Quickly stir in flour, salt, pepper and dill. Heat for 2 minutes, stirring constantly, then stir in broth. Bring to a boil and cook 3 to 5 minutes. Lower heat, and stir in cream and milk. Remove from heat and stir in sherry vinegar. Serve immediately.