Saute onions and garlic till translucent, about 5 minutes.
Add beef strips and brown and meld with the onions.
Add butter and mushrooms, saute for another 5 minutes.
Add vegetable broth, bring to a boil, deglaze the pan to get up all the good bits, and reduce to a simmer.
Meanwhile, boil noodles only until very al dente. Drain, and add to the beef mushroom broth mixture. Finish cooking the noodles in the broth.
Add ¾ of the package of sour cream and stir to combine completely. Save the other ¼ of the sour cream for dolloping on finished bowls of stroganoff.