Cook the pasta according to package directions. Drain and set aside.
Heat olive oil and butter in a large saucepan over medium heat. Add the sliced red onion and cook until soft and a little bit caramelized. Add the garlic and cook for a minute or so.
Add the tomatoes and toss everything for another minute. Add the chicken broth. Let this simmer for 5 minutes, until the broth has slightly reduced.
Add the tuna and the olives to the saucepan. Season with a little bit of salt. Add pepper to taste. Add the drained pasta to the saucepan and mix everything, making sure to coat the pasta with the sauce. Add more salt and pepper if needed. Turn off heat.
Add the cubed mozzarella and stir them in. Top pasta with chopped basil and serve.