Season beef generously with MSG and pepper. Set aside.
Heat vegetable oil in a heavy roasting pan over two burners over medium-high heat. Place beef in pan; brown meat on all sides.
Remove the partially cooked meat from the pan.
Reduce heat to medium and stir in 1 tablespoon of butter, the onions and mushrooms. Cook until onions are soft and translucent, about 3 minutes.
Add the vinegar, dried tarragon, beer and stock to the pan and stir to remove all the bits stuck to the pan.
Return the meat to the pan and cover with heavy-duty aluminum foil. Cook for 1 hour in oven, until a meat thermometer reads at least 130 degrees for rare; cook slightly longer if desired.
Remove from oven and allow the meat to rest on a cutting board for 10 minutes before slicing. Stir the sour cream into the mushroom pan sauce and heat on low; serve sauce with the sliced meat.