Tomato: Dip it into boiling water for a few seconds. Peel, slice, and remove seeds. Put aside.
Green peppers: Remove ribs and seeds and cut into lengthwise slices. Dip into boiling water for 30 seconds. Drain, and put aside.
Cucumbers: Peel and cut into very thin slices. Mix with ½teaspoon salt. Let stand for 1 hour, then squeeze, and discard the liquid extracted by the salt. Mix with apinch of white pepper and the vinegar, and put aside.
Mushrooms: Peel mushroom caps and cut into thin slices. Bring ½cup water to a boil and add half of the lemon juice and ½teaspoon salt. Cook mushrooms for 3 minutes. Drain, and put aside.
Asparagus tips: Clean the asparagus and cut the tips into halves. Bring 1cup water with ½teaspoon sugar and ½teaspoon salt to a boil. Cook till tips are done but still crunchy. Put aside.
Green beans: Cut beans into slanted (so-called French) slices. Bring ½cup water to boil with ½teaspoon salt, and cook beans till they are done but not soft. Put aside.
Boston lettuce: Wash in several waters and dry leaves with paper towels. Mix olive oil, remaining lemon juice, ½teaspoon salt, ½teaspoon sugar, apinch of white pepper, the tomato paste and parsley. Adjust salt and sugar to your taste. Toss lettuce leaves, taking care not to break leaves.
Arrange the lettuce leaves, already lightly coated with dressing, on a large flat serving platter, round or oval. Place cucumbers, asparagus tips, green peppers, beans and tomato slices in attractive separated mounds on the lettuce. Sprinkle mushroom slices over all. Chill the salad for about 30 minutes.