Mix tuna, onion, garlic, olive oil, vinegar, basil, and red pepper flakes into a bowl, add salt and pepper as desired. Spread bottom half of sandwiches with mustard, and top with a slice of cheese. Evenly spread the tuna mixture over the cheese, and cover with the desired amount of pickles. Place the top half of roll on top to close, and add butter to the outsides. In a skillet over medium heat, add sandwiches and press firmly down. Cook until the cheese is melted, about 3 minutes per side.