Sauté spinach and mushrooms in butter with salt, pepper and garlic. Remove from heat. In a bowl, whisk together milk and eggs. Place bread in bottom of a casserole dish. Add mushroom and spinach mix, then pour milk mixture over all. (Depending on how dry the bread is, you may need to make more milk/egg mix.) Cover dish with foil and bake at 300°F until cooked, about 30 min. (This recipe can also be made in 8-oz ramekins.)