Heat oil in a large saucepan over medium heat until shimmering. Add onion and salt, and cook until softened, about 5 minutes. Stir in coriander, cumin, ginger, pepper, cinnamon, and cayenne; cook until fragrant, about 2 minutes. Add tomato paste and garlic, and cook for 1 minute.
Stir in broth, water, and lentils, bringing to a vigorous simmer. Cook, stirring occasionally, until lentils are soft and about half have broken down, approximately 15 minutes.
Whisk the soup vigorously until it forms a coarse puree, about 30 seconds. Adjust the consistency with extra hot broth, as needed. Stir in lemon juice, and season with additional salt and lemon juice to taste.
Serve hot, garnished with cilantro, a drizzle of olive oil, and additional lemon juice if desired.