Heat oil to medium-hot; add London broil.
Brown all sides; save drippings.
Transfer meat to a roaster.
Sprinkle both seasonings over roast.
Place butter on top of roast.
Scatter peppers over roast/in pan.
Pour half of pepper juice over the roast.
Pour beef broth into the roaster.
Cover the roast; place in 300 F oven for 30 minutes.
After 30 minutes, reduce heat to 225-250 F.
Bake 7-9 hours; longer for larger cuts.
Check roast after 6-7 hours (you'll start to smell it). If fork tender, cut off oven and leave roast alone for a few more hours. If not, continue cooking until fork tender.
Steam potatoes and carrots and add to roast (submerge in juice) about 2 hours before roast is done.
