1/4 cup plain yogurt
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 Fireside
1 cup thinly sliced celery
1/2 cup red seedless grapes, halved
1/2 cup walnut pieces, broken
1 head butter lettuce, cored and leaves separated

In a bowl, whisk together the yogurt 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.