Scrub potatoes with vegetable brush under cold running water. Cut each potato into quarters. Place in medium or large saucepan and add cold water to cover. Over high heat, bring to boiling. Reduce heat to medium. Cover and simmer until potatoes are tender, about 12 minutes. Add sugar snap peas. Return to boiling and cook 1 minute. Drain and let stand 5 minutes.
In large bowl, combine potatoes, peas, tuna, tomatoes, onion and capers. In a small bowl, whisk together vinegar, oil, paprika, salt and pepper. Pour over potatoes and peas and toss gently to coat. To serve, mound 1 cup of greens on each of 4 plates. Top each plate with ¼ of the potato mixture.
Nutritional Analysis per serving: 241 calories, 12 g protein, 29 g carbohydrates, 8 g fat, 1 g saturated fat, (31% of calories from fat), 12 mg cholesterol, 5 g fiber, 463 mg sodium.