1. Combine first 4 ingredients in a small bowl. Cover; chill.
2. Cook potatoes in boiling water 10 minutes or until tender. Remove potatoes from water with a slotted spoon; set aside. Add green beans to boiling water; cook 3 minutes or until crisp-tender; drain. Rinse green beans under cold water; drain.
3. While potatoes and green beans cook, combine onion and vinegar in a bowl; toss well. Let stand 10 minutes. Drain onion; discard vinegar.
4. Combine potatoes, green beans, onion, tomatoes, and next 6 ingredients in a large bowl; toss gently to coat.
5. Arrange 2 cups vegetables, about 3 ounces crabmeat, and 3 shrimp on each of 4 plates. Spoon 2 tablespoons mayonnaise mixture into each lettuce leaf; place 1 leaf on each plate. Serve immediately with Italian breadsticks, if desired.
Insalata di Frutti di Mare (ihn-sah-LAH-tah dee FROO-tee dee MAH-reh) translated means seafood salad. It can be prepared with a variety of seafood, but we limited this version to crab and shrimp to keep it quick and easy.