You start with slicing a well-tenderised beef into thin stripes about 1.5 inch long. Slice an onion into long stripes as well. here is you can see the approximate amounts I took for 2-3 people:
Then coat the beef with flower. The flower is necessary to thicken the sauce, so be generous. I put two spoon in.
Sautee the onion first and the beef - second. You can use the same skillet, just evacuate the onion before loading the beef.
When the beef is brazed nicely, put the onion back in and cook a minute or two. This is a good time to put your dry spices in. I used a black pepper and cardamon.
While your beef is making the acquaintance with the onion, prepare the sauce base. Take two spoons of sour cream, SALT it (not the meat, the sour cream!) and dissolve it in the hot water. Make about a cup of mixture.
At this time you will notice that the flower, that we covered the meat with, started to stick to the bottom of the skillet because of all the moisture, coming from the meat and onions. It is normal and it is good. Using a plastic or wooden turner scrape the bottom of the skillet, at the same time pouring the sour cream base in. Keep the skillet on high heat, get a nice steam and see how your sauce thicken right on your eyes. Immediately turn the heat down and let the beef steam on low heat (very important, otherwise the sour cream will curdle). You can add some hot mustard in the sauce.