This was a very good recipe! Will definitely make again! I did add a few things, though.
1. I coated the beef in a big pinch of flour before I cooked it. (I also added the beef while the sauce was still simmering so the flour on the beef could help with thickening)
2. I added some garlic when I was sautéing the mushrooms and onions (I am obsessed with the stuff okay?)
3. I added a couple of splashes of Worcestershire when I added the sour cream and mustard. (In my experience most stroganoff’s have Worcestershire)
Thanks for reading! Let me know if you try it this way :)