Split croissants in half and place cut side up in one layer in a rectangular casserole.
Melt butter in a fry pan. Saute mushrooms and green onion until tender and liquid has evaporated. Set aside.
Beat together eggs and milk. Pour half over croissants, layer mushroom mixture and cheeses on top, then pour over remaining liquid. Position croissant tops over the bottoms, cut side down. Let stand overnight in the refrigerator. Bake at 350 degrees for 25 to 30 minutes or until set. Cover with foil if browning too quickly.