Place potatoes in a large saucepan; cover with water to 2 inches above potatoes. Bring to a boil; cook potatoes 6 minutes or until almost tender. Add beans; cook 4 minutes or until beans are crisp-tender and potatoes are tender. Drain; rinse with cold water. Drain.
Place potato mixture in a large bowl. Add tomatoes and next 5 ingredients (through tuna); toss. Drizzle potato mixture with oil and vinegar; toss to coat. Arrange 1 cup lettuce on each of 5 plates; divided potato mixture evenly among plates.
Wine note: A Spanish rosé, or Rosado, works well with the Spanish-Style Tuna and Potato Salad. Bodegas Borsao Rosado 2008 ($8) is crisp and dry, with herbal notes to match the tart dressing. -Jeffery Lindenmuth