In a gallon-sized bag, combine all ingredients for the roasted pork. Seal the bag and massage the marinade into the pork, ensuring an even coating. Marinate in the refrigerator for at least 2 hours, or for a more intense flavor, overnight. Occasionally massage the bag to redistribute the marinade. Before cooking, remove the bag from the refrigerator and let the pork reach room temperature.
Preheat the oven to 400 degrees. Place the pork in a Dutch oven with a lid and discard any excess marinade. Cook covered for 30 minutes, then uncover and flip the pork loin. Continue cooking uncovered until the internal temperature reaches 145 degrees, about 20–30 minutes more. Transfer the pork to a cutting board, cover with foil, and let the pork rest for 10–15 minutes before slicing.
Split the sub buns in half and butter the outer sides of both slices. Spread a thin layer of mustard on the inside of each slice. Layer with ham, sliced roast pork, pickles, and Swiss cheese, then place the top half of the bun over the fillings.
Place the sandwich in a grill pan over medium-high heat or use a panini press. Cook for 2–3 minutes per side. If using a grill pan, place a weight (like a cast-iron skillet) on top of the sandwich to press it down. Remove the sandwich from the grill and cut diagonally in half.
