Step 1:Prepare grill or broiler. Step 2: To prepare basil butter, combine 3/4 cup fresh basil leaves, 2 tbsp butter, softened, 1 tbsp fresh lemon juice, 1/4 tsp salt, 2 garlic cloves, minced in a food processor; process until smooth, scraping sides as needed. Set aside. Step 3:To prepare sauce, heat 2 tsp olive oil in a saucepan over med-high heat. Add 1/2 cup finely chopped red onion and 2 garlic cloves minced; sauté 3 minutes. Add : 3 cups grape or cherry tomatoes, halved; sauté 2 minutes. Stir in 1/2 cup dry white wine, 3 tbsp capers, 2 tbsp balsamic vinegar, and 1/4 tsp sugar; bring to a boil. Reduce heat; simmer 5 minutes, stirring occasionally. Stir in 1/4 cup chopped fresh flat-leaf parsley. Set aside. Step 4: To prepare tuna, sprinkle tuna with 1/2 tsp 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.