Split and clean kidneys; remove skin and tissue.
Soak in ice-cold water 1/2 hour.
Heat oven to 350°F.
Cut kidneys into 1/2-inch slices.
Melt butter in shallow skillet.
Saute kidneys in garlic butter 5 minutes.
Remove kidneys; discard garlic.
Add onions and mushrooms.
Saute until limp.
Add flour to skillet; stir to form paste.
Add liquids; bring to boil.
Layer kidneys and vegetables in buttered 2 1/2-quart casserole.
Cover with sauce.
Cover tightly; bake at 350 F 30 minutes or until tender.