Oh my goodness—it’s delicious. It might just be my favorite sandwich. If you’re a fan of tuna in any form, you’ve got to try this sandwich. Trust me: you’ll love it.
Season the tuna steaks on both sides with salt and pepper
In a medium bowl, toss together the red onion, capers, and basil, then use a fork to mix in the balsamic, Dijon, and mayo. Season with salt and pepper.
Roughly chop the tuna steaks, then add the mayo mixture and toss to combine.
Lightly butter the brioche buns, and toast on a griddle or under your broiler. Scoop ¼ of the tuna mixture onto each of the bottom buns, top with a piece of Swiss cheese and bake in the oven at 400 F until the cheese melts. Top with the toasted top bun and serve