Caramelize your onions by placing them in a large skillet with 2 TBSP olive oil and 2 TBSP butter. Cook over med/low heat for at least an hour, stirring regularly *please see linked post.
Dry rub the pork roast with the sugar & salt, place into large zip baggie and let sit for at least 1-2 hours (overnight is ok).
Crush the peppercorns with mortar/pestle, a spice dedicated grinder, or use a large grind from your pepper mill. Rub the meat with the peppercorns. Save what doesn't stick to add to marinate.
Melt butter in small pan over med/low heat. Add garlic, vinegar, thyme, and remaining pepper.
Preheat a large cast iron or other oven proof skillet on stovetop to med/high. Add oil.
Season the meat with a little salt and place into hot pan. Quickly brown all sides of pork.
Baste meat with butter mixture and place into preheated 375* oven. Set timer to 15 minutes. Turn pork over and spoon additional butter mixture over top. Set oven for another 20 minutes. Check doneness at this time. Thermometer should read to 145*
Thinly slice your pork loin.
To prepare sandwiches lightly grill/toast rolls with a little butter. Prepare some Sriracha mayonnaise (to taste). Layer toasted roll, mayo, a few thin slices of meat, and some caramelized onions.