In a Dutch oven or soup kettle, saute leeks and carrots in 3 tablespoons butter for 5 minutes or until tender. Stir in the broth, potatoes, dill, salt, pepper and bay leaf. Bring to a boil. Reduce heat; cover and simmer for 15-20 minutes or until potatoes are tender., Meanwhile, in a large skillet, saute mushrooms in remaining butter for 4-6 minutes or until tender. Discard bay leaf from soup. Stir in mushroom mixture. , In a small bowl, combine flour and cream until smooth; gradually stir into soup. Bring to a boil; cook and stir for 2 minutes or until thickened.