Brush pork with mustard; coat with brown sugar.
Brown pork on all sides in hot oil in a Dutch oven.
Transfer roast to a platter.
Add onion to Dutch oven; cover and cook 5 minutes, stirring occasionally.
Return meat to Dutch oven; add broth and pepper.
Cover and bring to a boil; remove from heat.
Bake at 325° for 1 hour.
Turn roast over; insert meat thermometer into roast, making sure thermometer does not touch bone or fat.
Cover and bake an additional 1 1/2 hours or until thermometer registers 170°.
Remove pork to a serving platter.
Combine cornstarch and water, stir ring until smooth; set aside.
Bring pan drippings in Dutch oven to a boil; stir in cornstarch mixture.
Return to a boil, and cook 1 minute or until thickened, stirring constantly.
Serve sauce over pork roast.