Using a small, sharp knife, peel away the tough outer strings from each celery stalk.
Thinly slice the stalks crosswise on the diagonal, then coarsely chop the leaves.
Peel the apple, quarter through the stem, and cut away the core. Starting from the short side, slice each quarter as thinly as possible. Working quickly, dump the slices into a bowl and toss them with the lemon juice, coating evenly to prevent browning.
Add the celery slices and leaves to the apple slices.
Spoon in the mayonnaise and cream.
Stir gently to coat the apple and celery. Top with the mint.
Transfer to a serving bowl, sprinkle with a pinch of salt and a few grinds of pepper, garnish with a mint sprig.