1Over medium high heat, gently cook beef tenderloin in 2 tablespoons of butter for about 2 minutes, until just seared on all sides. You will still be able to see red. Remove from pan and set aside in a rimmed dish or baking sheet so that you collect the juices.
2Return the pan to medium-high heat and cook the shallots and mushrooms in remaining butter until soft and wilted, about 5 minutes. Mix cornstarch into cold beef broth, whisk to blend. Pour into pan, and stir together with shallots and mushrooms until thickened, two or three minutes.
3Add sour cream and mustard, stir to blend. Add beef and juices from dish; stir over medium just till warmed through. Salt to taste.