1 small red onion (about 4 ounces), thinly sliced into rings
5 tablespoons red wine vinegar
1 3/4 pounds unpeeled russet potatoes
1/4 cup dry white wine
Freshly ground black pepper
1 pound large shrimp, cooked, peeled, and deveined, cut into 3/4″ pieces
3 plum tomatoes (about 2/3 pounds), quartered lengthwise, then crosswise
6 fresh basil leaves, thinly sliced
Place sliced onion in a strainer and rinse under cold water for 20 seconds; drain.
Transfer onion to a large bowl; add vinegar and toss to coat.
Meanwhile, boil potatoes in a medium pot of lightly salted water until tender, about 45 minutes.
Let cool slightly.
Using a kitchen towel, rub skins off potatoes.
Cut potatoes into 3/4″ pieces and transfer to a medium bowl; drizzle wine over.
Season with salt and pepper and gently toss to combine; set aside.
Add shrimp, tomatoes, and oil to bowl with onions.
Season with salt and pepper and toss gently to incorporate.
Let sit for 5 minutes until flavors meld.
Add basil, toss gently, and serve.