Dice 2 large Granny Smith apples into bite-size pieces, about 1/2-inch cubes. Place in a large bowl and drizzle with 1 tablespoon lemon juice. Add 1/4 cup granulated sugar and toss to combine so that the apples are well coated.
Add in 1 cup halved red grapes, 1 cup diced celery, and 1/2 cup chopped walnuts.
In a small mixing bowl whisk together 1/2 cup Greek yogurt, 1/4 cup mayonnaise, and 1/2 teaspoon salt until well combined.
Pour the sauce over the apple mixture and toss to coat thoroughly. Cover and refrigerate until ready to serve.