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 2 Tablespoons of unsalted butter.
Then distribute 1 cup of sliced yellow onion between both dishes. Now divided half of the seasoned potatoes amongst the dishes.
Slice 3-4 green onions and place them on top of the potatoes. Then add 1/2 cup Parmesan and 3/4 cup shredded mozzarella cheese. Now add the second half of the sliced potatoes. Lastly, sprinkle the remaining 1/2 parmesan and another 3/4 mozzarella, and pour 2 cups half and half right over the potatoes, so 1 cup per dish.
Cover with aluminum foil and bake at 400 degrees Fahrenheit for about 30 minutes. Then remove the foil and bake for an additional 30 minutes.