Waldorf Hotel's Waldorf Salad

1 cup walnut halves
1/2 cup mayonnaise
1/4 cup plain yogurt
1 teaspoon prepared mustard
Pinch of dry mustard
Juice of 1/2 lemon
4 to 6 tart apples, peeled, cored, and diced (2 cups)
1 to 2 cups finely diced inner ribs celery
(white part only), leaves reserved
Salt and freshly ground black pepper
2 bunches tender greens, such as arugula, baby kale,
or pepper cress, washed and dried
2 tablespoons olive oil
1 tablespoon fresh lemon juice
Preheat the oven to 325F.
Spread the walnuts on a baking sheet and toast in the oven
for 4 to 5 minutes, until aromatic and lightly toasted.
Let cool.
Combine the mayonnaise, yogurt, both mustards, and the lemon juice
in a large bowl. Fold in the apples and diced celery and
season with salt and pepper.
Put the salad greens in a large bowl. Add the olive oil and lemon juice,
season with salt and pepper, and toss well. Divide the greens among four
plates. Spoon the apple mixture onto the greens and sprinkle with the
toasted walnuts and reserved celery leaves.