Trim and discard pork surface fat. Cut meat apart following layers of fat around muscles; trim and discard fat. Cut into 2-inch chunks and rinse with cold water. Place in a 5- or 6-quart slow cooker. Turn heat to high.
Combine salsa, broth, onion and cumin seeds in a saucepan and bring to a boil over high heat. Pour over the meat. Add tomatoes and mix gently. Put the lid on and cook until the meat is pull-apart tender, 6 to 7 hours.
With a slotted spoon, transfer the pork to a large bowl; cover and keep warm. Pour the sauce and vegetables into a large skillet; skim fat. Bring to a boil over high heat. Boil, skimming froth from time to time, for about 20 minutes, to intensify flavors and thicken slightly. Add the pork and 1/4 cup cilantro; heat through.
To serve, ladle into bowls and garnish each serving with a dollop of sour cream and a sprinkling of the remaining 1/4 cup cilantro.Oven method: Total: 3 hoursPreheat oven to 350°F. Combine pork, salsa, 1/2 cup chicken broth, onion, cumin seeds and tomatoes in a 9-by-13-inch baking dish; cover snugly with foil. Bake until the pork is pull-apart tender, about 2 1/4 hours. Skim fat. Uncover and bake until the meat begins to brown, about 15 minutes more. Stir in 1/4 cup cilantro. Ladle into bowls, garnish with sour cream and remaining cilantro.
Ingredient Note: Tomatillo salsa (sometimes labeled salsa verde or green salsa) is a blend of green chiles, onions and tomatillos. It is sold in supermarkets.