Heat a medium skillet over medium-low heat. Toast the 1 cup walnuts, stirring frequently, until fragrant and golden. Remove from the pan immediately and roughly chop.
Combine 5 chopped apples, 4 stalks sliced celery, 1 pound halved grapes, and the toasted walnuts in a large bowl.
Combine 1/4 cup yogurt, 2 tablespoons mustard, 2 tablespoons tangerine juice, 1 tablespoon honey, and 1/2 teaspoon salt in a medium bowl. Whisk until incorporated and smooth. Taste and add more salt, honey, or tangerine juice if needed.
Pour dressing over the fruit mixture and toss to combine. Serve immediately or refrigerate for up to 1 hour.