Bring 2 qts of salted water to a boil.
Meanwhile, start browning the organic lean ground beef, breaking it up, and chop the onion.
Add the onion to the ground beef and stir regularly. When browned, if you did not use lean ground beef you will want to skim the fat off of the meat. Turn the heat to low and add 3 cups of beef stock and the beef base.
If the water has come to a boil, add the ribbon noodles and stir. Keep your eye on them so they don't boil over or overcook.
Wash and slice the mushrooms. Melt the butter in another pan and saute the mushrooms until golden, then remove from the heat.
Put the remaining beef stock in a shaker cup, add the flour, and shake until well blended.
Pour the flour/stock mixture through a sieve into the meat mixture, and stir well. (The sieve is the key to a gravy without lumps!) Turn the heat up to medium. It should thicken fairly quickly. Add the mushrooms and stir.
Taste the gravy at this point to see if you need to adjust the seasonings.
Add the sour cream and stir to blend. Ready to serve over the drained, hot noodles!
I served the stroganoff with simple wedged tomatoes and stir-fried zucchini. Love the colors!