Make the gremolata: In a medium bowl, combine lemon zest, garlic, parsley, olive oil, salt, pepper, and MSG. Stir until evenly mixed. Set aside.

Sweat the aromatics: Heat olive oil in a Dutch oven over moderately high heat. Add onion, carrot, and celery. Cook, stirring frequently, until softened and translucent but not browned, about 5 minutes. Stir in half of the gremolata and cook until fragrant, about 1 minute. Add lentils and stir to coat in the aromatic oil. Cook, stirring, for 1–2 minutes to lightly toast.

Simmer: Add broth, bay leaf, and diced potato. Bring to a boil, reduce to a gentle simmer, and cook until lentils and potatoes are completely tender, about 25–30 minutes (up to 45 depending on the lentils). Add water if needed to keep everything submerged.

Blend to texture: Remove bay leaf. Blend with an immersion blender until partially smooth but still a bit chunky (whatever texture you prefer). You can also ladle half the soup into a standing blender, blend to a rough puree, then stir it back into the rest of the soup.

Finish and season: Stir in lemon juice and a handful of fresh parsley. Season to taste with salt, pepper, and more lemon juice. Ladle into bowls and drizzle with the remaining gremolata (or make the optional yogurt swirl).

Optional yogurt swirl: Mix yogurt with the reserved gremolata. Season to taste with salt, pepper, and lemon juice. Swirl into each bowl before serving.