This might just be the ultimate tuna sandwich. Inspired by the classic salad, Mediterranean ingredients infuse this panino with tasty texture. It's even better if assembled an hour ahead.
In a bowl, toss together the tuna, fennel, olives, and capers. Add the lemon juice, mayonnaise, and basil and stir to blend. Season with salt and pepper.
On the bottom half of each roll, spread half of the tuna mixture, then layer each with half of the tomato, onion, and arugula. Cover with the top halves of the rolls. Wrap the panini tightly in aluminum foil a