5 apples (about 1 1/2 pounds), cored and large dice
4 stalks celery, cut on the bias into 1/4-inch thick slices
1 pound red seedless grapes, quartered
1/4 cup plain whole milk Greek yogurt
2 tablespoons spicy brown mustard
2 tablespoons freshly-squeezed tangerine juice (from 1 small tangerine)
1 tablespoon honey
1/2 teaspoon kosher salt
Heat oven to 350°F and arrange walnuts in a single layer in a large frying pan. Toast them in the oven until fragrant and golden, about 10-15 minutes. Let cool and roughly chop.
Combine apples, celery, grapes, and walnuts in a large bowl. Combine yogurt, mustard, tangerine juice, honey, and salt in a medium bowl. Whisk until incorporated and smooth. Taste and add more salt if needed.
Pour dressing over the fruit mixture and toss to combine. Serve immediately or refrigerate for up to 1 hour.