In medium saucepan, combine potatoes and enough water to cover. Bring to a boil. Reduce heat; cook 10 to 12 minutes or until tender. Drain; rinse with cold water to cool. Drain well.
In large bowl, combine cooked potatoes, spinach, tomato, onion and 1/4 cup of the dressing; toss gently. Arrange salad on 2 serving plates. Spoon tuna onto center of each salad. Drizzle each with 2 tablespoons remaining dressing.