In a saucepan, combine the crème fraîche, milk, and 2 sticks (7.75oz/220g) butter with the rosemary, sage, and garlic. Bring to a simmer for 2 minutes, remove from the heat, and let infuse for 15-20 minutes.

Meanwhile, place the potatoes in a large pan and cover them with water. Add a generous pinch of salt, bring to a boil, and cook for about 20 minutes, until the potatoes are tender. Drain and mash with a potato masher, ricer, or food mill.

Remove the herbs and garlic from the cream mixture and pour over the mashed potatoes. Season with salt and pepper and stir to blend.

Preheat the broiler and transfer the mashed potatoes to a flameproof casserole or baking dish with a depth of at least 2 inches (5cm). With the back of a large spoon, smooth the top and make a design of your choice.

Melt the remaining 1 tablespoon (209) butter and drizzle over the potatoes. Place under the broiler for 8-10 minutes, until the potatoes are nicely browned. Serve hot.