Serve with broad egg noodles because they’ll catch all the luscious sauce! Stroganoff is made with a tender cut of beef so you don’t braise the meat for a long cooking time like you do for a stew … in this case if you do the slices of tenderloin will be dry.
In a large saucepan or skillet, combine cold beef stock, flour and red wine. Whisk over high heat until mixture is just coming to the boil. Add the thyme sprigs and tomatoes. Reduce heat to medium and simmer uncovered until mixture is reduced to 3 cups, about 30 minutes.
In a bowl, season sliced beef with salt and pepper. Toss to combine.
In a Dutch oven or large skillet, heat 2 tbsp. of butter over high heat until sizzling. In batches, brown beef well, about 2 to 3 minutes. Be careful not to overcrowd the pan. Remove beef and reserve. Reduce heat to low add remaining 2 tbsp. butter to Dutch oven. Saute onions and mushrooms until soft, about 3 to 5 minutes. Add the garlic and saute for 1 minute. Add nutmeg, paprika, chile flakes, mustard, Worcestershire sauce and reduced stock and wine. Cook uncovered over medium heat for 10 more minutes, just to develop the flavours. Sauce should be slightly syrupy.
Add back the reserved beef and stir in the sour cream continue to cook over medium heat until just heated through. Adjust the seasoning. Sprinkle with chopped parsley. Serve with egg noodles.