In a large skillet, sauté 16 oz of sliced mushrooms until they become tender.
Using a sharp knife or mandolin, thinly slice 16 Yukon potatoes. Place the sliced potatoes into a bowl and season with 1 1/2 tsp salt, 1/2 tsp black pepper, and 1/2 tsp garlic powder.
Generously grease 2 baking dishes or oval gratins with unsalted butter.
Then add the sliced yellow onion to both dishes. Now divided half is the seasoned potatoes amongst the dishes.
Slice the green onion and place it on top of the potatoes. Then add half of the Parmesan and mozzarella cheese. Now add the second half of the sliced potatoes. Lastly sprinkle the remaining parmesan, mozzarella, and pour the half and half right over the potatoes.
Cover with foil and bake at 400 degrees Fahrenheit for about 30 minutes. Then remove the foil and bake for an additional 30 minutes.