Preheat your oven to 400 degrees
Add 3 cups of heavy cream, 3/4 cup of finely grated parmesan or parmigiano reggiano, 3/4 cup of finely grated Gruyère, 4 cloves of finely minced garlic, 2 teaspoons of salt and 1/2 teaspoon of black pepper in a large bowl and mix until combined
Add the thinly sliced potatoes and toss until all of the slices are completely cooked
Prepare a deep 2 quart baking dish by greasing the interior surface with 2 tablespoons of butter
Arrange the potatoes in rows, packing them in tightly and uniformly
Cover the baking dish tightly with foil
Bake on the center rack of a 400 degree oven for 45 minutes
Transfer the baking dish onto a heat proof surface (such as a wooden cutting board) *Note: leave the oven door shut and oven set at 400 degrees
Remove the foil and sprinkle the remaining parmesan and gruyère over the potatoes
Return the baking dish to the oven and bake at 400 degrees for an additional 25 minutes or until the potatoes are very tender
Once the potatoes are soft and the cheese is melted and beginning to brown, transfer the baking dish onto a cooling rack
Allow the gratin to cool for about 10 minutes before serving
Garnish with additional shredded parmesan and finely chopped parsley (optional)