Cut the beef into 1 to 2-inch cubes. Pat dry and season with (a little) kosher salt and (lots of) pepper.
Heat about 3 tablespoons of canola oil or olive oil in a heavy bottomed dutch oven over high heat. Brown the meat on all sides (you might need to brown the meat in batches). Lower the heat to medium and set the meat aside.
Add 3 tablespoons of butter to the pot and scrape the bottom of the pan with a wooden spoon to try to release any browned bits of meat. Take care not to let the butter burn - you might have to lower the heat more. Add 1 pound of sliced mushrooms, 3 cloves of chopped garlic, a little more salt & papper, and some thyme and cook until the mushrooms are browned and cooked through, about 5-10 minutes. Then set the mushrooms and garlic aside with the beef.
Deglaze the pan again, this time with about 2 tablespoons of cognac or brandy, scraping with a wooden spoon until the alcohol has burned off. Add the 3 cups of beef stock, thyme, and browned beef back into the pot, and cook, covered, over very low heat for 2 hours.
Remove the beef again and dissolve 1 tablespoon of Dijon mustard and 2 tablespoons of sour cream into the stock. Add salt and pepper to taste, and fold in the beef, mushrooms, and garlic.
Serve over buttered lightly-salted egg noodles.