Preheat oven to 350 degrees. Season beef generously with Ac’cent and pepper. Heat vegetable oil in a roasting pan over two burners over medium-high heat.
Place beef in pan; cook until brown on all sides. Remove the partially cooked meat from the pan. Reduce heat to medium, and stir in 1 tablespoon of butter and the onions and mushrooms. Cook until onions are soft and translucent, about 3 minutes.
Add the vinegar, dried tarragon, beer and stock 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, until an instant-read thermometer reads 130 degrees for rare.
Remove from the 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, so as not to curdle before serving with the sliced meat.