On a decorative platter, layer and arrange the watercress, then the tomatoes, and the onions. Sprinkle the cheese over the onions.
In a small bowl, whisk together the garlic, mustard, vinegar, salt, and pepper. Gradually add the oil, whisking, until it emulsifies. Dress the salad with the vinaigrette, and top with the fresh parsley. Serve with French bread croutons.
Yield: 4 servings