In hot skillet cook onion and garlic in hot oil 2 minutes. Add tomatoes, olives, capers, salt, and pepper to skillet. Bring to boiling; reduce heat. Cover and simmer 3 minutes. Stir together wine or broth and cornstarch; add to skillet. Cook and stir until thickened and bubbly. Cook and stir for 2 minutes more. Stir in basil. Makes about 4 cups sauce.