Cook the asparagus in boiling salted water until crisp-tender, about 8 minutes. Drain and refresh in cold water. Cool. Cut each artichoke bottom into 6 equal-sized pieces. Peel the avocadoes and slice thinly. Sprinkle the avocadoes with the lemon juice so that they do not discolor. Make the vinaigrette: Whisk the vinegar, salt and pepper and oil together to blend. In a salad bowl toss together the avocadoes, radishes, artichoke bottoms, asparagus, mesclun, Boston lettuce and watercress. Pour in the salad dressing and mix well.
