I make a version of this salad every winter, a couple of times or more. For the first several years, I used celery root to top the beluga lentils. Celery root is velvety and divine, but it's a bitch to peel.
This year I happened upon the lucky merger of celery and fennel bulb, because that was what was on hand. (Not that I've never combined celery with fennel; it was just a new twist on an old lentil recipe). Along with some thinly sliced mild onion, it makes a stellar wintry raw melange, airy and light and tart atop the gently cooked beans.
A simple, very simple dressing of first-rate olive oil, vinegar (I used homemade apple-pear vinegar), salt and pepper ties the two layers together.
Come on. I think you can do this without a recipe.