Prepare grill or broiler.
To prepare basil butter, combine first 5 ingredients in a food processor; process until smooth, scraping sides as needed. Set aside.
To prepare sauce, heat oil in a saucepan over medium-high heat. Add onion and 2 garlic cloves; sauté 3 minutes. Add tomatoes; sauté 2 minutes. Stir in wine, capers, vinegar, and sugar; bring to a boil. Reduce heat; simmer 5 minutes, stirring occasionally. Stir in parsley. Set aside.
To prepare tuna, sprinkle tuna with 1/2 teaspoon salt and pepper. Place tuna on grill rack or broiler pan coated with cooking spray. Cook 5 minutes on each side or until desired degree of doneness. Serve with sauce and basil butter. Garnish with basil leaves, if desired.