Heat 2–3 tbsp oil in a large pot and sauté onion for 4 minutes until translucent. Add turmeric and garlic, cook for 3 more minutes until lightly browned.
Stir in tomato paste and cook for 3–4 minutes until it darkens and smells aromatic.
Add a bit more oil, then mix in the flour and sauté for 1 minute over medium heat.
Add lentils, 6 cups boiling water, salt, cinnamon, pepper (and optional potatoes). Bring to a boil, cover, then simmer on low for 1 hour, stirring often.
After 45 minutes, check thickness—simmer uncovered if it’s too watery. Adjust salt to taste.
Serve hot with toasted bread or basmati rice.