Put the pork roast in a covered casserol and roast at 275 for about 2 or 3 hours until done using your meat thermometer to check. I will often put it in frozen - no real need to thaw it first. As long as the pork is cooked through, it's fine. I don't even put any water in the dish, but you can put a half inch of water if you want to, it won't hurt the roast. When done, remove from oven and let cool. Save the juice! When cool enough to handle, dice the pork and throw away the fatty parts. Save the bone if any.
In a very large stock pot (at least 2 1/2 gallon capacity), add diced pork and juice from the pork and sprinkle with flour - tossing to coat. Cook on medium low until it gets a little sticky from the flour. Add remaining ingredients and bones. Simmer at least one hour - stirring occaisionally. Do not let the bottom burn. If it seems too thick add water or chicken broth. Remove the bone.
I use this for so many things! I smother my burritos, huevos, or eat like a soup with tortillas. I also pour it over burgers (see Gourmet Green Chile Burgers).