Wash and dry the celery; pull existing threads. Cut the sticks into thin slices.
Wash and dab the apples, remove the stones. Cut the apples into fine cubes and drizzle with lemon juice. Mix the mustard with the olive oil and the wine vinegar. Salt sparingly, pepper generously.
Arrange the apple cubes and the celery slices in a salad bowl. Add the sauce and mix well.