Preheat the oven to 275°F. Line a small roasting pan with aluminum foil. 

In a small bowl, stir together the cumin, garlic powder and paprika. Rub the spice mixture all over the pork. Sprinkle with salt and pepper. Place the pork in the prepared pan, cover the meat with the onion and pour in the water. Cover the pan tightly with aluminum foil and roast until the meat is very tender, 5 to 6 hours. Let the pork rest until cool enough to handle. 

Using 2 forks, shred the pork into bite-size pieces. Skim the fat off the juices in the roasting pan. This can be made up to 2 days in advance. Cover and refrigerate the pork and juices separately. 

In a large pot over medium-low heat, warm the vegetable oil. Add the onion and cook, stirring occasionally, until translucent, about 5 minutes. Add the garlic and cook, stirring often, until fragrant, about 2 minutes. Add the fresh tomatoes and stir until softened, about 2 minutes more. Stir in the broth, beans, hominy, canned tomatoes, oregano and cumin and bring to a boil. Reduce the heat to low, cover and simmer, stirring occasionally, for 30 minutes.

Add the reserved pork to the pot and simmer, uncovered, for 30 minutes more to allow the flavors to meld. Season with salt and pepper and add the reserved cooking juices from the pork, if desired. 

Divide the posole among bowls. Garnish with cheese, lime wedges cilantro and a dab of sour cream.  Serve with flour tortillas.