Prepare the vegetables: Peel or clean the carrot, onion, garlic, and celery, if using, and roughly chop. There’s no need for pretty or even pieces – the soup will be blended later anyway. Peel and roughly chop the potato and set aside separately.
Sauté the vegetables: Heat the olive oil in a pot and sauté the carrot, onion, garlic, and celery, if using, over medium heat until the onion becomes translucent.
Add the spices and tomato paste: Add the cumin, paprika, tomato paste, mint, parsley, vegetable stock powder, and a little chili powder, if using. Stir well and cook for 1–2 minutes.
Rinse the lentils: Meanwhile, rinse the red lentils thoroughly in a fine-mesh sieve under running water until the water runs mostly clear.
Cook the soup: Add the lentils, potato, and water and bring to a boil. Simmer for about 15 minutes, until the lentils and vegetables are soft.
Blend the soup: Blend until smooth using an immersion blender directly in the pot or transfer to a regular blender. If the soup is too thick, gradually add a little more water until it reaches your preferred consistency.
Season to taste: Add the lemon juice, salt, and black pepper and adjust to taste.
Serve: Serve the Turkish red lentil soup hot, topped with lemon wedges and sumac, if desired. Fresh, soft bread is perfect on the side.