Birria is the Mexican version of shredded beef in red sauce.
Remove stems and seeds from chilies and roughly chop onions and garlic. Bring the sock to a boil and add onion, garlic, and chilies. Return to boil then reduce to simmer. Simmer until onion and chilies are soft.
Meanwhile, cut beef into 2-3 inch chunks and brown on all sides. Then move beef into crockpot or instapot along with any pan juices.
Blend onion, garlic, chilies, and stock then strain into pot with beef. Use the back of a spoon to help as much liquid through as possible. Stir, getting as much of the beef under the sauce as possible.
Set instapot for meat/stew or 20 min with pressure and allow pressure to release naturally. Crockpot, high 4 hours or until meat shreds easily.
Shred and add salt to taste.