In a large soup pot, heat the olive oil over medium-low heat.
Add the onions, garlic, leeks, salt, pepper, and thyme and cook, stirring occasionally, until the vegetables are very tender, about 15 minutes.
Add the celery and carrots and cook until they begin to soften, about 10 minutes.
Add the chicken stock and 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, about 1 hour.
Stir in the lemon juice and season with salt and pepper to taste.
Serve hot with a drizzle of olive oil and a sprinkling of lemon zest.
