Place a large pot of water on high heat and bring to a boil. In a separate large saucepan combine oil, 2 tablespoons butter, and the flour over medium-high heat. Whisk well to eliminate clumps. Add the sliced onions and stir for 2-3 minutes.
Add the mushrooms, garlic, 2 tablespoons thyme, and 1/2 teaspoon salt. Move around the pan continuously for 5 minutes or so, until the mushrooms have started to soften. Then add cognac, beef stock, and Dijon mustard. Mix well, and let the sauce simmer another 15-30 minutes. (Until reduced to a dark color with a thin gravy texture.)
Meanwhile, preheat the grill or a skillet to high heat. Rub the steaks with oil, salt, pepper and thyme. Grill 3-5 minutes per side, depending on thickness (1 1/2 inches = 5 minutes per side.)
Boil the noodles according to package directions. After draining, add 2 tablespoons butter, 1 tablespoon parsley, and salt to taste. Cover and set aside.
After the steaks have rested 5 minutes, slice into thin strips. Add the sour cream to the sauce and whisk well. Salt and pepper the sauce to taste. Immediately plate the noodles topped with sauce. Then lay the steak across the top. Garnish with extra parsley!
Notes
This Easy Beef Stroganoff Recipe with Butter Noodles includes sour cream to give the sauce a tangy note and silky texture but is not thick. If you like a thick sauce, you can double the amount of flour.