In a salad bowl, combine the watercress and witlof leaves. Quarter the apple, remove its core and thinly slice the fruit. Sprinkle the apple with some of the lemon juice and add the apple slices to the salad greens.
In a small bowl, whisk together the remaining lemon juice, vinegar, mustard and olive oil, then adjust the seasonings with salt and black pepper, to taste. Drizzle the dressing over the salad and serve.