Cook the broccoli rabe in a separate pan, not in the soup pot. That way, any excess bitterness from the vegetable goes down the drain with the cooking water rather than into the soup. You'll find, though, that a bit of bite enlivens the lentils. Warming Soup Recipes
How to Make It
In a large saucepan, heat the oil over moderately low heat. Add the onion and cook, stirring occasionally, until translucent, about 5 minutes. Stir in the carrots, celery, and garlic and cook, stirring occasionally, until the vegetables start to soften, about 5 minutes.
Stir in the lentils, water, tomato paste, bay leaf, salt, and pepper. Bring to a boil. Reduce the heat and simmer, partially covered, stirring occasionally, until the lentils are tender, about 30 minutes.
Meanwhile, in a large pot of boiling, salted water, cook the broccoli rabe until just tender, about 5 minutes. Drain.
Stir the lemon zest into the soup. Remove the bay leaf. Ladle the soup into bowls and top each serving with some of the broccoli rabe.
Notes
Variations
Omit the broccoli rabe. Shred ten ounces of raw spinach, escarole, or Swiss chard leaves and stir them into the soup five minutes before it's done.
The lentil soup is also delicious on its own, without any greens at all.