This is one of my favorite dishes to make. My original recipe was from Betty Crocker, but by now (almost 40 years later), it is just my own. If you use tomato paste (and I do), you do not need sugar. It is sweet enough. My husband's family are of eastern European background, and he used to make it a more pure white/brown sauce. It was good, but he likes mine better. He jokes (but it is true) that it is a cross between real stroganoff and goulash. To make it easier (that is to say less likely for the sour cream to separate), I take the sour cream, tomato paste, and a bit of Wondra flour, and mix them in a bowl together. Then I add them to the meat, onion, garlic, mushroom, off the heat, then gradually re-warm the mixture until it thickens.
My husband makes a great French onion soup, and we always have leftovers. I often use the leftover soup as part of my stroganoff.