Step 1

Heat the oil in a large nonstick skillet over medium-high. Add the mustard to the skillet; cook, stirring constantly, 15 seconds. Add the leeks, salt, and 1/4 teaspoon of the pepper; cook, stirring often, until the leek has softened slightly, 3 to 4 minutes. Transfer the leek mixture to a 5- to 6-quart slow cooker. Add the stock, carrots, and millet to the slow cooker. Cover and cook on HIGH until the carrots are very tender, about 6 hours. Cool the soup 10 minutes.

Step 2

Transfer one-third of the soup to a blender. Remove the center piece of the blender lid (to allow steam to escape); secure the blender lid on the blender. Place a clean towel over the opening in the blender lid (to avoid splatters). Blend until smooth. Pour into a large bowl. Repeat the procedure with the remaining soup. Stir in the milk, lemon zest, lemon juice, and remaining 1/4 teaspoon pepper. Ladle the bisque into bowls; sprinkle evenly with the chives before serving.