Preheat oven to 325°F
Pat rump roast dry
Season all sides with salt, pepper, and optional garlic powder, onion powder, and rosemary
Heat oil in a large oven-safe skillet or Dutch oven over medium-high heat
Sear roast on all sides until browned
Remove roast from pan
Add sliced onions, carrots, and celery if desired
Place roast back in pan
Add beef broth or water to the pan
Cover tightly with a lid or foil
Roast in the oven until tender, about 3 to 4 hours
Check internal temperature with a meat thermometer
Remove from oven when it reaches 135°F for medium-rare or 145°F for medium
Let rest 15 to 20 minutes
Slice thinly against the grain
Serve with pan juices or gravy