I don’t remember the origin of this salad but it’s a favorite in our house.
For dressing: Whisk together 1/2 teaspoon Dijon mustard, 1 tablespoon freshly squeezed lemon juice, and freshly ground black pepper, to taste. Slowly whisk in 2 tablespoons extra-virgin olive oil.
For salad: Quarter and core (do not peel) 1 Granny Smith apple, slice thinly, and place in serving bowl with 3/4 teaspoons freshly squeezed lemon juice; toss to coat. Peel strings from 3 large stalks celery, cut into julienne strips and add to the apples. Julienne 2 heads of Belgian endive and add to the apples and celery.
To serve: Drizzle dressing over vegetables and toss. Season with salt and pepper, to taste. Serve immediately.