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 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 stand for 15 minutes.
