Heat the beef stock with the carrot, 3 thyme sprigs, and bay leaf. Pat the beef dry and season it with salt and pepper. Heat 3 tablespoons oil in a large heavy bottomed skillet over high heat. Fry the meat in batches so that it is browned on all sides. Lower the heat to medium and return all the meat to the pot. Add the onions and cook until they are soft, about 5 minutes. Pour in the cognac and cook until the alcohol has burned off, about 5 minutes. Add the beef stock, discarding the carrot, thyme sprigs, and bay leaf. Cook, partially covered, over a very low flame for 1 1/2 to 2 hours.
In a large skillet over medium heat, melt 3 tablespoons butter in the remaining 3 tablespoons olive oil. Add the mushrooms, garlic, and remaining 3 thyme sprigs and cook until the mushrooms are browned and cooked through. Remove from heat and set aside.
When the meat is done, remove it from the heat and fold in the mushrooms, sour cream, mustard, and parsley. Taste and adjust the seasoning with salt and pepper.
Meanwhile, cook the noodles in a large pot of boiling, salted water until tender. Drain the noodles well, toss with the remaining 2 tablespoons butter, and season with salt and pepper. Serve the stroganoff over the noodles; garnish with more sour cream and chopped parsley.