Heat the olive oil in a large pot over medium heat. Add the chopped onion and cook until soft, about 5 minutes.
Add the minced garlic, ground cumin, ground coriander, and turmeric. Cook for another minute, stirring constantly.
Add the rinsed red lentils, vegetable broth, diced tomatoes, and salt. Bring to a boil, then reduce heat and let simmer for 25-30 minutes, or until lentils are tender.
Stir in the lemon juice and chopped cilantro. Adjust seasoning if needed. Serve hot.