Preheat oven to 350°F.
Score the fat on top of pork and place fat side down in cold roasting pan. Place pan over medium heat and brown well on both sides.
Remove pork to paper towels and brown onion slices in pork fat. Now discard all fat, add butter, return pork, and add apples and water.
Cover pan and bake for 2 to 2 1/2 hours, or until an instant-read thermometer inserted into the meatiest section registers 150 to 155°F.
Uncover for last half-hour of cooking and add a little water if apples have dried out. Let the roast rest 5 to 10 minutes before slicing. (Internal temperature will raise to 160°F during this time.)