Beef Stroganoff

2 lbs. cubed beef
1 onion, diced very fine
1 cup sour cream
1 qt. beef stock
2 tsp. garlic, chopped
1 tsp. tomato paste

Coat beef in flour, salt and pepper.  Heat saucepan to medium high heat.  Add 1/4 cup oil and sear meat until browned on all sides.  Remove meat and reserve.  Add onions and garlic to pan, saute until tender.  Add tomato paste, slightly brown.  Return meat to pan and add stock.  Bring to boil and simmer about 1 hour or until meat is tender.  Add sour cream, salt and pepper.  Thicken with cornstarch and water.  Off heat add canned mushrooms if desired.