Thinly slice onions and mushrooms.
Cut sirloin into chunks
Season and flour sirloin (shaking in a bag is easiest)
Saute onions and mushrooms until just softened.
Saute beef until just browned and crusty in oil; cook in batches if needed. (Use dutch oven or other oven safe pan and continue with same pan for baking to make less dishes).
Combine onions, mushrooms, stock, and Worcestershire into a dutch oven/casserole and bake at 300* for 2-3 hours; until beef is very tender.
15 Min. before serving fold in sour cream.