Season beef all over with salt and pepper. In a cast-iron pan, add oil. When hot sear all sides of beef. Remove beef. Add onions to pot and saute for 3 to 5 minutes. Add tomato paste, chili flakes and pepper; stir for 30 seconds. Add stock and soy sauce and bring to a boil. Return beef to pot. Cover with foil.
Roast in oven, turning once after 1 hour, until beef is very tender, 2 1/2 hours. Remove beef from pot. When cool enough to handle, place on a cutting board and shred with forks.