Combine beef stock, brown sugar, soy sauce, vinegar, Worcestershire and Italian seasoning. Marinate London broil roast overnight, 2 days for bolder flavor.

Preheat broiler or grill. Remove roast from marinade, reserving marinade. Pat roast dry, rub with oil, season with salt and pepper and place on broiler pan; broil 15 minutes. Flip and broil an additional 12-15 minutes for medium rare (when internal thermometer registers 125°F). Allow roast to rest 10 minutes on carving board, tented with foil.

Bring reserved marinade to a boil in a small saucepan, reduce heat and simmer 10 minutes, reduce sauce by half. Whisk in butter.

Slice roast very thinly against the grain; serve with warm sauce.