Pat pork dry and season with salt and pepper. Brown in oil in a large skillet in 2 batches for 4 to 5 minutes per batch, turning to color the broad sides.
Put onion in a 6-quart slow cooker and arrange pork pieces over it. Whisk mustard, vinegar, honey, broth, Worcestershire and garlic powder and pour around the meat.
Cover and cook on low for 6 to 7 hours until fork-tender and at least 145°F, usually around 190 to 200°F when the shoulder connective tissue softens. Keep pieces intact rather than shredding.
Lift pork carefully to a platter and rest for at least 3 minutes. Transfer onion and cooking liquid to a saucepan and skim excess fat.
Simmer for 8 to 12 minutes until the mustard sauce lightly coats a spoon, then spoon it over the pork pieces and serve.