I make this soup every Halloween and is the 'signal' to the family that the cool weather has arrived. I've been making it for 15 years now and out of all the soups I make, it is still everyone's favorite. Enjoy!
Melt 3 tablespoons butter in a large saucepan over medium heat. Mix in leeks and carrots, and cook 5 minutes. Pour in broth. Season with dill, salt, pepper, and bay leaf. Mix in potatoes, cover, and cook 20 minutes, or until potatoes are tender but firm. Remove and discard the bay leaf.
Melt the remaining butter in a skillet over medium heat, and saute onions and garlic until clear add the mushrooms saute 5 minutes, until lightly browned. Stir into the soup.
In a small bowl, mix the half-and-half and flour until smooth. Stir into the soup to thicken. Garnish each bowl of soup with fresh dill to serve.