Trim and rinse the fennel bulb. Cut it in half lengthwise, thinly slice each half into semicircles, and transfer to a serving bowl. Rinse and dry the arugula or mesclun, tear the leaves into bite-sized pieces, add them to the bowl, and top with the red onion slices and grapefruit sections. Cover the bowl with a damp towel and chill until ready to serve.
In small bowl, whisk together all of the dressing ingredients until creamy. Refrigerate the dressing in a glass jar until ready to serve. Just before serving, pour the dressing over the salad and toss gently to distribute well.
From Moosewood Restaurant Daily Special (veganized).