Finely slice all of the potatoes and onions into thin coins. Use a cheese grater to freshly shred the Gruyere Cheese.
Then, spray a baking dish with cooking spray. Line the potato coins vertically in the dish leaning against one another. Evenly distribute the onions between the potatoes.
Heat a sauce pan to medium heat. Add butter & flour and keep whisking until it’s absorbed. Pour in milk and whisk again until no lumps remain. Add onion dip mix and cheese.
Pour the onion cheese sauce over the sliced potatoes and onions letting it seep between the layers.
Cover the dish with foil and bake for 1 hour. Remove the foil from the potatoes and add the remaining ½ cup of shredded gruyere cheese bake for 30 more minutes.
Garnish with more green onion, salt + pepper, and serve hot!