In a bowl, combine all burger ingredients, except the mozzarella. Divide the mixture evenly, form patties, and then chill for at least 15 minutes.
Heat olive oil on a skillet on medium heat. Add a few patties at a time, and cook for about 5 minutes on each side. Once golden brown and crispy, place on toasted buns. Top with hot sauce, mayo, arugula, and red onion.