Combine all ingredients except mushroom filling. Place half of meat mixture in a lightly greased 10x6x2 in. baking dish; form into loaf shape.
Make a shallow well lengthwise in meat. Spoon mushroom filling into well. Spoon remaining meat mixture over filling, covering all of filling and shape into loaf. Bake at 350 degrees about 1 hour. Stand 5 minutes before cutting.
• This could be a good meat loaf with tomatoes and tomato sauce added, admitting mushroom filling.