In a small mixing bowl, mix yogurt, mayonnaise, lemon juice, salt, and pepper together.
In a large bowl, toss together chicken celery, apple, grapes, walnuts, and the dressing. Spoon into lettuce cups and serve immediately.
*Use leftover rotisserie chicken or you can bake, grill, or cook it in the Instant Pot. Place a (8-10 ounce) chicken breast in the bottom of the slow cooker with 1/4 cup water. Sprinkle with some onion salt, paprika, and pepper. Cook on high pressure for 10 minutes with a 5 minutes slow release followed by a quick release.