Prepare the Produce. Start by dicing the apples into small bite-sized chunks. No need to peel them unless you prefer it. Halve the grapes and slice the celery thinly. Place everything in a large mixing bowl.
Toast the Walnuts. In a dry skillet over medium heat, toast the walnuts for 3–5 minutes, stirring often, until fragrant. Let them cool slightly, then roughly chop.
Make the Dressing. In a small bowl, whisk together the mayonnaise, Greek yogurt, lemon juice, salt, and pepper until smooth and creamy.
Combine. Pour the dressing over the apple mixture and toss to coat everything evenly. Add the toasted walnuts and gently fold them in to keep their crunch.
Chill and Serve. For best flavor, chill the salad for at least 15 minutes before serving. Serve on its own, over lettuce leaves, or as a side with chicken or turkey.