Toast the Walnuts. Place the chopped walnuts in a small, dry skillet over medium heat. Toast for 3-5 minutes, tossing frequently, until fragrant and lightly browned. Immediately remove from the skillet and set aside to cool completely.
Emulsify the Vinaigrette. In a jar bwith a tight-fitting lid, combine the extra-virgin olive oil, lemon juice, lemon zest, nonfat yogurt, Dijon mustard, minced garlic, and black pepper. Seal the jar and shake vigorously for 30 seconds until the dressing is creamy and fully combined.
Marinate the Foundation. In a large mixing bowl, combine the rinsed and drained cannellini beans, chickpeas, kidney beans, and the thinly sliced red onion. Pour about two-thirds of the prepared vinaigrette over the bean mixture and gently toss to coat. Let this stand for at least 10 minutes to allow the beans to absorb the flavors.
Build the Salad Core. Add the halved tomatoes and diced cucumber to the marinated beans. Gently fold the ingredients together until just combined.
Fold in the Greens. Add the arugula to the bowl. Drizzle the remaining vinaigrette over the top. Using tongs, gently toss the salad until the arugula is lightly wilted and everything is evenly coated in the dressing.
Garnish and Serve. Just before serving, sprinkle the toasted walnuts, crumbled feta, chopped dill, and chopped parsley over the top of the salad. Serve immediately for the best texture.