• Combine lemon zest, parsley, garlic and half of the olive oil in a medium bowl and stir with a fork until homogenous. Season to taste with salt and pepper and set aside.
• 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).
• 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.
• To serve, drizzle extra lemon-parsley mixture on top of each serving. Soup can be stored in the refrigerator for up to 5 days.