8 Ounce smoked Swiss or smoked Gouda cheese, sliced
For the pork, preheat the oven to 425° F. Place the pork in a small roasting pan. Brush with oil; sprinkle with salt and pepper. Roast for 50 to 60 minutes until the center is slightly pink.
For the jam, combine the sugar, vinegar, and honey in a medium saucepan. Simmer until the sugar is dissolved. Add the onions and cook over medium heat until the onions are tender and golden, and most of the liquid has been absorbed, 20 to 25 minutes. Set aside to cool.
For the aioli, combine the mayonnaise, lemon juice, garlic, horseradish, cayenne, and olive oil in a small bowl. Set aside.
To assemble the sandwiches, slice the pork very thinly. Toast the buns, and spread both of the cut sides with aioli. Top with arugula, several slices of pork, the jam, and cheese. Serve warm, or grill as a panini.