Split croissants in half and place bottoms cut side up in one layer of an 11 x 7 inch casserole dish.
Melt butter in a fry pan. Saute mushrooms and green onion until tender and liquid has evaporated. Spoon over croissant halves.
Beat together eggs, milk and seasoned salt. Pour over mushroom mixture. Put a spoon of Swiss cheese and a spoon of Parmesan cheese over each croissant. Put 1/4 cup of Mozzarella cheese over each croissant. Position croissant tops over the bottoms, cut side down. Cover with foil and refrigerate overnight.
Remove casserole from refrigerator one hour before baking. Heat oven to 350 F and bake for 20 minutes. Remove foil and bake another 20 minutes until set.