In large soup pot, heat oil over medium heat. Sauté onions, garlic, red pepper flakes, cumin and coriander until onions are soft, about 5 minutes.
Sort and rinse lentils and add to pot along with tomatoes and vegetable stock. Bring to a boil; reduce heat to low and simmer, covered, until lentils are tender, about 40 minutes.
Before serving, stir in lemon juice, cilantro and salt.