Toast the pine nuts. Place them in a dry skillet over medium heat and toast, shaking often, until golden brown and fragrant. Remove from the pan and let cool.
Prep the fennel. Trim the stalks and tough root end from the fennel bulb, reserving some of the fronds for garnish. Slice the fennel as thinly as possible with a sharp knife, mandoline, or food processor.
Prep the apple. Core the apple (no need to peel) and slice into thin matchsticks.
Assemble the salad. In a large bowl, combine the fennel, apple, raisins, and feta.
Make the dressing. In a small bowl, whisk together the Greek yogurt, lemon juice, salt, and pepper. Pour over the salad and toss gently to coat.
Finish and serve. Sprinkle the toasted pine nuts on top and garnish with a few fennel fronds. Serve immediately or refrigerate for up to 3 hours before serving.