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
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.
This is a great time to turn to an old friend, Beaujolais. The wine's frank fruitiness, soft texture, and lively acidity will contrast with the lentils and will blunt the bitterness of the broccoli rabe.