Heat oven to 400°. Season pork with salt and pepper and drizzle with olive oil. Place on a roasting tray and roast until a thermometer inserted reads 160°, about 45 minutes. Remove from oven and transfer to a cutting board to rest, about 15 minutes. Thinly slice the pork.
Halve the bread lengthwise and spread the insides of each roll with 1 tablespoon mustard. Divide the ham, pork, cheese, and pickles evenly among sandwiches.
Heat half the butter in a 12-inch skillet over medium. Working in batches, cook two sandwiches, pressing down on the top with a 10-inch cast iron skillet, until golden on the bottom, about 3 minutes. Flip the sandwiches over, press with the skillet, and continue cooking until the other side is golden brown and the cheese has melted, about 3 minutes more. Transfer to a cutting board and cut in half on the diagonal to serve.