Directions: In a large saut, pan, heat the 3 T. butter with the onion. Add the beef sirloin, that has been cut into thin strips. Brown on all sides. Season with salt and pepper and set aside. To make the sauce, in a large saucepan, melt the 2 T. butter and stir in the flour to create a roux. Whisk until smooth. Slowly stir in the beef stock and simmer until thickened. Add the sour cream, alternately with the tomato paste, whisking constantly to combine. Add browned meat to sauce and simmer for 30 minutes. Serve over cooked fettucini and garnish with chopped parsley.