Make just enough of this recipe. Use the portion planning tool to calculate exactly how much of each ingredient you’ll need.Heat the oil in a sauté pan over medium to high heat until almost to the smoke point. Sauté the beef quickly, until well browned but overcooked. It should still be slightly pink inside. If necessary, sauté in several batches or in more than one pan to avoid overcrowding the pan.Remove the meat from the pan and discard excess fat.Place the butter in the sauté pan and sauté the onions and mushrooms briefly without letting them brown.Add the white wine. Reduce by half over medium to high heatStir the tomato paste and mustard, and then add the demi glace. Reduce over heat until lightly thickened. Stir in the sour cream. Season to taste with salt and pepperKeep this sauce hot.Add the sautéed beef back to the mixture and simmer for a few min. Stir the meat and make any adjustments in seasoning.Add in the pasta and serve immediately.  You can garnish with a little dollop of sour cream.