1. Pulse onion in a food processor until finely chopped, then transfer to a bowl. Add tomatoes with juice to processor and pulse until chopped.
2. Heat oil in a 12-inch heavy skillet over high heat until hot but not smoking. Toss shrimp with 1/2 teaspoon salt and 1/4 teaspoon pepper, then add to skillet and saute, turning over once, until just cooked through, about 3 minutes total. Transfer shrimp to a plate with tongs (do not clean skillet).
3. Force garlic through a garlic press into skillet and cook, stirring, until golden, about 15 seconds. Stir in Sherry, scraping up any brown bits from bottom of skillet, then add onion and remaining 1/4 teaspoon salt and 1/4 teaspoon pepper and cook, stirring occasionally, until all of liquid is evaporated and onion is just tender, 5 to 6 minutes.
4. Add tomatoes and olives and simmer, stirring occasionally, until sauce is slightly thickened, 5 to 7 minutes.
5. Stir in shrimp with any juices accumulated on plate and heat through. Season with salt.

Yield: 4 servings