First, pound the meat and cut it in 1 cm thick stripes against the grain. Remember that will make the meat more tender. You can pound the meat with a cast iron skillet.
Cut the onions and garlic in little cubes, and set them aside. Wash and slice the mushrooms and set aside
Take a pan and put the vegetable oil, when the oil is very hot add the meat stripes and spark salt to the meat meanwhile you fry it, for about 1-2 minutes until is brown on both sides. Depending how thick you cut your meat before.
Remove the meat and let the oil in the pan where the meat was cooked,and add the onions, and cook them till they are transparent.
Add the champignons to the onions, if they get a bit dry add a little bit more oil. When the champignons are brown add the garlic and the flour and stir all well for 1 minute and add the beef broth and let it set
Let it cook till the sauce start to get creamy, add the sour cream, the mustard and sugar and mixed all well with a spoon. Finally, add the meat and let all cook together for about 5 minutes. When is ready, add salt and pepper to your taste and you are done! Serve with fresh pasta.