Rinse cannellini beans until runoff is clear, drain 2 minutes and blot gently on a clean towel.
Remove stalks, outer layers and core; slice the bulb crosswise about 1/8 inch thick.
Drain and blot roasted pepper, then cut into bite-size strips.
Combine shallot, lemon zest and juice, Dijon, maple syrup, garlic, salt and pepper; rest 5 minutes, then whisk in oil.
Fold beans, fennel and roasted pepper into vinaigrette and rest 20 minutes.
Taste a bean with fennel and adjust lemon by the teaspoon, salt by a pinch or oil by the tablespoon.
Immediately before serving, lift arugula and torn basil through the bean base without compressing leaves.
Scatter toasted walnuts over the salad after tossing.
