Rinse celeriac with care, trim root and leaf base. Place celery on a cutting surface and trim the skin from top to bottom. Quarter celery, cut into cubes about 1 cm. Immediately sprinkle with half of the lemon juice. Cook in lightly salted water for about three minutes. Drain, rinse with cold water. Drain and cool.
Rinse apples, pat dry, grate and perhaps remove peel. Then cut into quarters, remove the core from each. Cut apples into narrow wedges, cut each wedge diagonally in half. Sprinkle with remaining juice of one lemon.
Drain pineapple pieces well. Coarsely chop walnuts.
Mix all prepared salad ingredients in a large baking bowl. Stir through sour salad mayonnaise, pineapple juice, whipping cream, sugar, salt and pepper. Season to taste. Pour sauce over lettuce, mix well. Cover and let stand for two or three hours.
Season the salad again. Garnish with mint leaves, if desired.
Season leaf salad repeatedly. Dress with mint leaves to taste.