Grate the gruyere and cheddar cheeses and place into the refrigerator until ready.
In a bowl, add the heavy cream, the minced garlic, and the melted butter and combine well using a whisk. Set aside
1 1/2 cup Heavy cream, 2 cloves Garlic, 2 tbsp Butter
Preheat the oven to 350 degrees Fahrenheit
Peel the potatoes and using a mandoline or knife, slice potatoes 1/8″ thick and place slices into a large pot or large bowl with cold water
2 1/2 lbs Russet potatoes
When all the potatoes are sliced, drain them completely and pat dry
Layer 1/3 of the thinly sliced potatoes in a 9×13 inch casserole dish or similar baking dish
Pour 1/3 of the heavy cream mixture, 1/3 of the salt and pepper and thyme, and 1/3 of each of cheddar and gruyere cheese over the potatoes
1 1/2 cup Sharp cheddar cheese, 1 tsp Kosher salt, 1/2 tsp Black pepper, 1 1/2 tsp Thyme, 1 1/2 cup Gruyere cheese
Repeat this process for the second and third layer but on the third layer do not add the shredded cheese
Cover with aluminum foil and bake for 75 minutes or until the center potatoes are tender (use a knife to test)
Remove the au gratin potatoes from the oven and remove foil. On the top layer of potatoes add the remaining cheese and return to the oven
Bake for an additional 15 minutes or until the cheeses are golden brown and the dish is bubbling
Remove from the oven and allow to rest for at least 10 to 15 minutes before serving to allow to set up
Serve. Salt and pepper to taste