Preheat oven to 275-300 degrees F, depending on your ovens temperment.
Season 1 (4-5 lb) bone-in pork roast with salt and pepper. Use more or less in order to cover roast.
Place roast in a covered roast pan (I like to use the blue speckeled metal roasters). Bake for approximately 2 hours.
Remove pan from oven and place onion slices on top and sprinkle with whole all spice, about 10-15 balls. Add approximately 1/2" water to pan, cover.
Continue to cook roast in oven for 3 more hours. Roast will be falling apart but still moist.
For gravy: Remove roast from pan. Heat juice in bottom of pan till boiling (if not stove top friendly, transfer to cooking pan). Mix together about 1/4 cup of flour and 1/2 cup water or more to form a fairly thin thickening agent and slowly add to boiling juice, stirring vigorously. Once desired consistancy is met, remove from heat and strain into gravy boat. Salt and pepper to taste. You may want to spoon some of the grease off the top or use a gravy divider.