Step 1

Combine lemon zest, parsley, garlic, and half of olive oil in a medium bowl and stir with a fork until homogenous. Season to taste with salt and pepper and set aside.

Step 2

Heat remaining olive oil in a large Dutch oven over medium-high heat until shimmering. Add leek, onion, carrot, and celery and cook, stirring frequently, until softened but not browned, about 5 minutes. Add half of parsley-lemon mixture and cook, stirring, until fragrant, about 1 minute. Add lentils and stir until coated in oil. Add bay leaves and stock. Bring to a boil, reduce to a bare simmer, cover with lid slightly cracked, and cook until lentils are completely tender and falling apart, about 1 hour, adding water as necessary (lentils should be fully covered at all times).

Step 3

Using a hand blender, blend soup until as smooth as desired. Alternatively, transfer half of soup to a standing blender, blend until smooth, and fold back into remaining soup. Whisk in lemon juice and season to taste with salt and pepper. Serve, drizzling extra lemon-parsley mixture on top of each serving. Soup can be stored in the refrigerator for up to 5 days.