Adjust oven rack to center position and heat oven to 350 degrees Fahrenheit.
Rub bottom and sides of shallow baking dish with garlic.
Mince remaining garlic and set aside.
Once garlic in dish has dried, about 2 minutes, spread dish with half the butter.
Bring half-and-half, salt, pepper, nutmeg, cayenne , potatoes, and reserved garlic to boil in medium saucepan over medium-high heat, stirring occasionally with wooden spoon.
Reduce heat and simmer until liquid thickens, about 2 minutes.
Pour potato mixture into prepared dish; shake dish or use fork to distribute potatoes evenly.
Gently press down potatoes until submerged in liquid; dot with remaining butter.
Bake until top is golden brown (basting once or twice during first 45 minutes) for about 1 hour 15 minutes.
Let rest 5 minutes and serve.