Boil each vegetable separately until just tender. Remove with a slotted spoon and put into a large bowl of ice cold water.

Peel and cut the potatoes, carrots and apple into small cubes and place them together with beans in a large bowl.

Add lemon juice, salt and pepper to the mayonnaise and mix well.

Pour the lemon mayonnaise over the vegetables and mix gently.

Break the lettuce up into large leaves and use to line a salad bowl. Spoon the vegetable mixture on the top of the leaves.

Refrigerate for several hours.