Combine the fennel, rosemary, oregano, garlic salt, 1 teaspoon kosher salt and a generous amount of pepper in a small bowl.
Slice the pork tenderloins in half crosswise and rub evenly with the spice mixture. Heat 3 tablespoons olive oil in a large cast-iron skillet over a medium-high heat. Add the pork and sear, turning, until golden brown on all sides, about 5 minutes. Place the skillet in the oven and bake until the pork is cooked through and a thermometer inserted in the middle registers 140 degrees F, 18 to 20 minutes.
To toast the ciabatta rolls, place a dry skillet on a medium heat until it is hot, about 5 minutes. Slice the rolls in half and place the rolls in the skillet cut-side down until they are toasted how you like them, 15 to 30 seconds. Set aside.
Remove the pork to a cutting board and let rest for 5 minutes before slicing to your desired thickness.
Grate the garlic clove and combine it with the mayonnaise and lemon zest in a small bowl. Season with black pepper.
Add the arugula to a bowl, then drizzle over the remaining 3 tablespoons olive oil and the balsamic vinegar and toss to combine.
Generously spread the garlic mayonnaise on the tops and bottoms of the rolls. Place some pork slices on the bottom halves, then add slices of roasted red pepper and a generous amount of the arugula. Place the other halves of the rolls on top and serve.