Prepare the Pot: In a Dutch oven, combine the onion, whole jalapeño, and water.
Prepare the Pork: Carefully cut the pork shoulder along the bone and butterfly it open to expose the center.
First Seasoning: Rub the inside of the butterflied pork with half of the brown mustard. Season it with half of the salt, all of the crushed red pepper, sliced garlic cloves, black pepper, oregano, and chicken bouillon powder.
Second Seasoning: Roll the pork back together. Season the outside of the pork with the remaining brown mustard, salt, and garlic powder.
Roast: Place the pork, fat side up, over the onions in the Dutch oven. Cover the pot and roast in a preheated oven at 375°F for 3-4 hours, or until the bone can be easily removed. Remove the lid for the last hour of cooking.
Shred: Allow the pork to rest in the pot with the onions and jalapeño before shredding.