Combine leeks, broth, 1/2 teaspoon rosemary, and garlic in a large saucepan over medium-high heat; bring to a boil. Reduce heat, cover, and simmer 10 minutes. Discard bayleaf. Place leek mixture in a blender; process until smooth.
Place flour in pan. Gradually add the milk, stirring with a whisk until blended. Add pureed leeks and ground nutmeg; stir to combine. Bring to a boil. Reduce heat; simmer 5 minutes, stirring constantly. Remove from heat, and stir in 1/4 teaspoon rosemary, butter, salt, and lemon rind.
I loved the asparagus version of this soup, but this one needed a lot of doctoring. I stirred in a full 1/2 TB of "better than bouillon" chicken soup based, some smoked paprika & white pepper, along with some white wine and chopped green onions. The it wasn't bad, but I think it probably needs another cup of chopped leeks. Will try with four cups next time and see if that gives it more of a leek flavor and makes it thicker, too.
This was surprisingly good, considering how simple it is. It had good leek flavor and a nice creamy consistency. I made a few changes to fit our tastes: replaced rosemary with a combo of tarragon and thyme and omitted nutmeg. I used an immersion blender. It's probably easy to double this recipe.