Heat the olive oil in a large pot, add the onion, celery, and garlic, and sauté for 2 minutes.
Add the cumin and cook, stirring, for one minute longer.
Add the tomato paste, stir until smooth, and add the stock and lentils.
Bring just to a boil, reduce the heat, and simmer, partially covered, for 20 minutes, or until the lentils are very soft.
Add 1 cup of water if the soup seems too thick.
Puree in batches in a food processor or blender, return to the pot, and stir in the lemon juice.
Season with salt and pepper.