Cut the steak into thin 1/4 inch strips. Heat oil and 1 tablespoon of butter over medium-high heat on a skillet. Add the sirloin steak strips and stir-fry for 1 minute, or until browned. Remove the cooked steaks and keep aside.
Melt the remaining butter in the skillet and reduce the heat from medium-high to medium. Add mushrooms and chopped onion and sauté for 3 minutes or until softened. Add flour and cook 1 minute, stirring constantly.
Gradually add in stock and Worcestershire sauce. Stir constantly and cook 3 minutes or until mixture begins to simmer. Reduce heat to low and cook 10 minutes.
While the sauce cooks on low heat, bring water to boil in a pot with salted water and cook pasta 8-10 minutes or until tender, but firm. Drain pasta.
Add the stir-fried sirloin steak strips to mushroom sauce and cook 2 minutes, or until meat is heated. Remove the skillet from the stove and stir in sour cream. Pour sauce over pasta and serve immediately.