In a large soup pot, heat the olive oil over medium-low heat. Add the onion, garlic, celery, carrot, and cumin, and cook, stirring occasionally, until the vegetables are tender, 10 to 15 minutes.
Add 5 cups water and the lentils to the pot. Increase the heat to high, cover, and bring to a boil. Uncover, reduce the heat, and simmer, stirring occasionally, until the lentils are tender, 45 minutes. Some will fall apart during cooking.
Stir in 2 tablespoons lemon juice, and season with salt, pepper, and more lemon juice to taste. Serve hot, drizzled with olive oil and sprinkled with lemon zest, dill, and feta (if using).
