Preheat oven to 375 degrees. In a small bowl combine garlic powder, onion powder, paprika, and salt and pepper. Rub on all sides of the pork loin. Place pork loin onto a baking dish, fat cap side up. Bake for 50-60 minutes, or until meat thermometer reaches 145 degrees. Let roast rest for at least 10 minutes before slicing. Slice rested roast into 1/2-inch slices.
While the pork is cooking place the sliced yellow onions and 2 tbsp olive oil in a medium skillet, and cook over med-low heat for 40-45 minutes, or until golden brown, stirring occasionally. Season with salt and pepper. Remove from heat until sandwich assembly.
While the onion and pork are cooking, make the Dijon dressing. Drizzle about 1 tbsp of olive oil into a small sauce pan, add minced shallots and cook over medium heat for 1-2 minutes. Stir in minced garlic and cook for another 1 minute, remove from heat. Whisk in honey, avocado oil, apple cider vinegar, Dijon mustard, yellow mustard, and salt and pepper. Set aside until ready to assemble the sandwiches.
Once pork is done roasting, reduce oven temp to 350 degrees. Spray a large Bundt pan with non-stick cooking spray. Remove the dough from the 2 tubes. Connect both tubes of dough to make a ring. Place dough ring into the Bundt pan, gently press down and around so the dough sets into the pan. Bake for 30-35 minutes or until golden brown.
While the bread is still in the Bundt pan, brush with melted butter and sprinkle with kosher or coarse sea salt. Remove from pan and do the same on the other side. Allow bread to cool for about 15-20 minutes before slicing.
Using a serrated bread knife, slice the bread in half horizontally. Place sliced provolone onto the top half, sliced side up, place under the broiler for about 3 minutes or until cheese is melted. Brush the bottom half with the Dijon dressing. Arrange pork slices on top of the Dijon, followed by caramelized onions, and finally arugula. Season the entire sandwich with additional fresh cracked black pepper. Top with the cheese bread topper, slice and serve.
Serve with additional honey Dijon for dipping.
Roast could be made a day ahead, and sandwich could easily be made with leftovers, as it only uses about half of the roast for the sandwiches.