Use tomatoes held at room temperature until fully ripe. Cut a thin slice from the bottom of each tomato and discard; then cut remaining tomatoes into 4 slices each; set aside. In a small bowl, mix oil, lemon juice, mustard, salt and sugar until well blended. In a medium bowl, toss greens with about 1 tablespoon of the dressing. On each of 4 plates, place a bottom tomato slice; sprinkle lightly with cheese and greens, repeat layering 3 more times ending with the top tomato slice; drizzle with remaining dressing. Serve as an appetizer or salad, with grilled beef or chicken, with cheese and fish, etc.