Cheesy Broccoli-Potato Soup

Peel broccoli stems and chop into 1-inch pieces. Transfer 1 cup stems to a medium bowl and set aside. (Reserve any remaining stems for another use.) Cut broccoli florets into bite-size pieces. Transfer to a separate medium bowl and set aside.

Heat butter in a large heavy pot over medium heat until melted. Add leek, pepper, salt and the reserved broccoli stems. Cook over medium heat, stirring occasionally, until the stems are starting to soften, about 8 minutes. Add garlic and cook, stirring constantly, until fragrant, about 1 minute.

Add broth, milk and potatoes to the pot. Bring to a boil over medium-high heat; reduce heat to medium-low and simmer until the potatoes are tender, about 12 minutes. Add the reserved broccoli florets and cook, stirring occasionally, until bright green, about 3 minutes. Remove from heat and gradually stir in 1 ½ cups Cheddar.

Working in batches, if necessary, pour the soup into a blender. Secure the lid on the blender and remove the center piece to allow steam to escape. Place a clean towel over the opening. Process until smooth, 45 seconds to 1 minute. (Use caution when pureeing hot liquids. Alternatively, blend the soup in the pot with an immersion blender until smooth.) Return the soup to the pot; stir in vinegar. Serve immediately, topped with the remaining ½ cup Cheddar and garnished with chives, if desired.