Position a rack in the lower third of the oven and preheat to 350 degrees . In a large saucepan, melt the butter over medium heat. Add the onion and garlic and cook until softened, about 5 minutes. Add the rosemary and bay leaf and cook until fragrant, about 1 minute. Stir in the half-and-half and bring to a boil. Remove from the heat, stir in cheese and let stand for 15 minutes.

Peel and thinly slice the potatoes. Discard the rosemary sprigs and bay leaf from the cream mixture and bring to a simmer. Stir in the potatoes, sour cream, 1 1/2 teaspoons salt, 1/2 teaspoon black pepper and the cayenne and cook until the potatoes are slightly softened, about 10 minutes.

Transfer the mixture to a greased 8-inch square baking dish. Bake until golden-brown, about 45 minutes. Let cool for ten minutes before serving.