Lightly brown mushrooms and onion in butter. Remove from heat; mix in sour cream. Set aside.
Combine meat loaf ingredients; mix well. Shape half meat mixture in oval, place in shallow baking pan.
Lengthwise down center make a shallow well, spoon prepared filling from first step into well.
Shape remaining meat mixture, covering filling and sealing bottom and top meat mixtures together.
Bake at 350 degrees for 50 minutes.
Top with prepared glaze. Return to oven (uncovered) for an additional 10-15 minutes or until glaze is as desired. Remove from oven. Let stand 5 minutes before slicing.