Preheat the oven to 375 degrees F (190 degrees C). Spray a baking sheet with cooking spray.
Mix tuna, bread crumbs, egg, black pepper, and garlic salt together in a bowl; fold in diced tomatoes and onion. Form mixture into 4 patties; place onto the prepared baking sheet.
Bake in the preheated oven until burgers are golden brown and crisp, about 10 minutes per side.