Remove from the heat and let steep for 15 minutes, then discard the rosemary, sage and garlic.
Add a generous pinch of salt and simmer over moderate heat until tender, about 20 minutes. Drain well, then pass the potatoes through a ricer into the pot. Fold in the cream mixture and season generously with salt and pepper.
Scrape the potatoes into a 12-inch round flameproof pan or baking dish (2 inches deep) and, using a spoon, decoratively swirl the top. Gently brush with melted butter.