1/4 cup mayonnaise
1 tablespoon apple cider vinegar
1/3 to 1/2 cup buttermilk
1/2 cup crumbled blue cheese
1/4 cup grated yellow onion
1/4 teaspoon salt
1/8 teaspoon freshly ground black pepper
Pinch cayenne
2 cups diced sweet red apples, such as Gala or Jonathan (2 to 3 medium apples)
1 cup thinly sliced celery
1/2 cup red seedless grapes, halved
1/2 cup walnut pieces, broken 
1 head Bibb lettuce, cored and leaves separated

In a bowl, whisk together the mayonnaise and vinegar. Slowly add 1/3 cup buttermilk, whisking to thicken. Add the blue cheese, onion, salt, pepper, and cayenne, and whisk well to combine, adding more buttermilk, as desired. Set aside. 

In a large bowl, combine the apples, celery, grapes, and walnuts. Add the dressing, to taste, and toss well. Arrange the lettuce leaves on 6 salad plates. Top with the tossed salad and serve.