I typically use tenderloin, but tri-tip or a really good sirloin would work great as well. The point is to make sure it is tender, because a tough piece of meat will ruin your stroganoff!
Reduce heat to medium and add the mushrooms, onions and garlic to the pan; sauté until the aromatics (onions and garlic) are soft and the mushrooms have released much of their liquid and started to brown.
Add the sherry to de-glaze the pan and cook for about 1 minute, then add the beef broth, cream of mushroom soup and the browned beef cubes.