Prep your ingredients. Dice the onion, carrots, and celery.
Mince the garlic. Rinse the lentils under cool water and pick out any debris.
Sauté the aromatics. In a large pot, warm 2 tablespoons olive oil over medium heat. Add onion, carrots, and celery with a pinch of salt.
Cook 6–8 minutes, stirring occasionally, until soft and lightly golden.
Bloom the spices. Add garlic, tomato paste, cumin, smoked paprika, and coriander. Stir for 1–2 minutes until fragrant and the tomato paste darkens slightly.
Add lentils and liquid. Stir in the lentils, then pour in 6 cups vegetable broth (or water). Add the bay leaf and a small pinch of red pepper flakes if using.
Bring to a boil.
Simmer gently. Lower the heat and simmer uncovered for 25–35 minutes, stirring occasionally, until the lentils are tender but not mushy. Add more liquid if you prefer a looser soup.
Season and brighten. Remove the bay leaf. Add 1 teaspoon salt and black pepper to taste.
Stir in the zest of 1 lemon and the juice of half to a whole lemon, adjusting to your taste.
Finish with parsley. Stir in a generous handful of chopped parsley. Taste and adjust salt, pepper, and lemon.
Optional texture tweak. For a creamier body, blend 1–2 cups of the soup and pour it back in, or use an immersion blender for a few quick pulses.
Serve. Ladle into bowls and finish with a drizzle of olive oil. Add a pinch of sumac or a spoonful of yogurt if you like.
Warm crusty bread on the side is perfect.