Gently pat the beef slices thinner, trying to give them an oblong oval shape. (about 1/4 inch in depth). Season with salt and pepper. In a large skillet, heat a tablespoon of butter and a tablespoon of oil over medium high heat. Add the beef slices and fry until golden brown on both sides, turning once. Transfer the meat onto a place and put aside.
If neccessary, add another spoonful of butter onto the skillet. Add flour and cook for a minute, stirring carefully. Now and the beef stock and stir, until you've got a thin gravy. Return the meat into the pan and simmer for 10-15 minutes, until the meat is cooked.
When beef is tender, then use a slotted spoon to remove them from the pan and keep warm on a plate, covered with a piece of foil.
Add the sour cream to the sauce, cook for a few minutes, then season to taste with salt and pepper.
While the beef is cooking, prepare the onions. Peel the onions, halve lengthwise and then cut into thin slices. Melt another spoonful of butter and a spoonful of oil in a large skillet over medium heat. Add the onions and cook, stirring occasionally, until golden. This will take about 10-15 minutes.
To serve: spoon the gravy onto a plate, top with a slice or two of beef and garnish with a generous amount of fried onions.