In very large (5-quart) glass bowl, place about half of the lettuce. Layer with onion, chicken, zucchini, cheese, salami, remaining lettuce and the tomatoes.
In small bowl, mix mayonnaise, mustard and horseradish; spread over tomatoes, spreading to edge of bowl.
Cover and refrigerate at least 8 hours but no longer than 24 hours. Garnish with additional cherry tomatoes and parsley or watercress if desired.