Potatoes Gratin

classic Potato Gratin from Bon Appetit

5 cloves garlic
1 T unsalted butter, room temp
2 medium shallots, quartered through root ends
2 ½ c. heavy cream
1 T. kosher salt
1 tsp black pepper
1 T thyme leaves
4 lbs russet potatoes, scrubbed, very thinly sliced
5 oz. Gruyere cheese, grated
1 oz. Parmesan, finely grated

Preheat oven to 325°F.  Cut 1 clove garlic in half and rub the inside of a 3-qt. baking dish with cut sides. Smear butter all over inside of dish.  Bring Shallots, cream, salt, pepper, thyme, and remaining 4 cloves garlic to a simmer in a small saucepan over low heat; cook until shallots and garlic are very soft, 15-20 minutes. Let cool slightly.  Transfer to a blender; blend until smooth.

Arrange potato slices in prepared dish, fanning out a handful at a time. Shingle as you work until bootom of dish is covered. Tuck smaller slices into any gaps to fill. Pour cream mixture  over potatoes and cover dish tightly with foil. Bake potatoes until tender and creamy, 60-75 minutes. Let cool.

Place rack in highest position heat broiler. Remove foil and top potatoes with cheeses. Broil until bubbly and golden brown, 5-10 minutes.