Step 1

Preheat oven to 400°F.

Step 2

Add the ½ stick of butter to a sauce pan over medium heat and melt. Add the garlic and allow to simmer for 1 minute. Add the cream and herbs to the pan and bring to a simmer. Remove the cream mixture from the heat and set aside.

Step 3

Peel, then slice the potatoes and sweet potatoes into 1/4-inch-thick rounds. Add the potatoes to a large mixing bowl and toss with salt and pepper.

Step 4

With a tablespoon of butter, grease a 13x9x2-inch baking dish. Arrange sliced potatoes in rows overlapping in the pan and alternating between the two potato types.

Step 5

Pour the cream mixture over the potatoes, pressing lightly to submerge the potatoes into the liquid.

Step 6

Sprinkle on the cheese, cover with tin foil and bake for 40 minutes. Then uncover and bake until top of gratin is golden and most of liquid is absorbed, about 10-15 minutes longer. Let stand 10 minutes then serve.