Beef Stroganoff

I have been cooking a lot lately and enjoying some experimentation with new recipes.  This one is from the Joy of Cooking, but adapted slightly.  Although this is one I have made before, it has been a really, really long time.  It is so delicious and I have received great reviews.  My DH said he expected to have it again soon.  So we did when we invited some coworkers of mine and their spouses.  It was a lot of fun, and I love cooking for a crowd!
Without further ado, Beef Stroganoff:

1 1/2 lbs beef tenderloin/top loin/sirloin trimmed and cut into pieces (I just use stew meat because it's faster... haha)
Salt and pepper
1 Tbsp butter
1 thinly sliced onion

1 1/2 Tbsp butter
1 Tbsp all purpose flour
1 cup beef stock (I usually use a bullion cube)
3Tbsp sour cream
Salt/pepper to taste

For the sauce:
In a small saucepan over medium heat, melt 11/2 Tbsp butter.  Add flour and whisk until smooth.  Add 1 cup beef stock, whisking constantly to prevent lumps.  Simmer until the sauce is smooth and thickened.  Set aside and keep warm.

Meat: Over medium high heat, melt the 2 Tbsp butter and saute the onions and add the beef.  Cook until evenly browned.  (It can remain a little pink in the center) *My recipe says to discard the onion, but I usually leave them in because I love them so much... :)

Return the sauce to medium heat and warm, but do not boil, and add the sour cream and salt and pepper to taste.  Add the meat and serve immediately.  Serve over rice or egg noodles (my favorite).

Notes:  I quadrupled the sauce when I had company (6 people total), just because that's the best part and it doesn't make a whole lot of it to begin.  So just be aware you might want to make more gravy.