1/4 cup mayonnaise
1 tablespoon freshly squeezed lemon juice
1 boneless, skinless chicken breast (about 8 ounces), cooked and cubed
1 medium sweet apple, such as Fuji or Pink Lady, cored and diced
1/2 cup red seedless grapes, halved
1/2 cup thinly sliced celery (2 to 3 medium stalks)
1/2 cup toasted walnuts, coarsely chopped
Salt
Freshly ground black pepper
Butter lettuce leaves, for serving
Place the mayonnaise and lemon juice in a large bowl and stir to combine. Add the chicken, apple, grapes, celery, and walnuts, and fold into the dressing. Taste and season with salt and pepper as needed. Serve in the lettuce leaves as a salad or as lettuce cups.