From It Must've Been Something I Ate (Canada, UK), by Jeffrey Steingarten.
4 Tbs. butter (1/2 stick)
1 cup (scant) milk
1 large garlic clove, peeled and lightly crushed
1/2 tsp. freshly ground white pepper
3/4 tsp. salt
1/8 tsp. freshly grated nutmeg (about a dozen gratings)
1-1/2 tbs. baking potatoes
1-1/2 cups heavy cream
A large, low baking dish made of enameled iron, glass, or earthenware. The quantities in this recipe work out perfectly when baked in a dish measuring about 120 square inches on the inside bottom, where the slices of potato will lie. This translates into a rectangle 9 by 13 inches, or 10 by 12 inches; an 11 inch square; a 12 inch circle; or an oval 10 by 15 inches. An enameled iron baking dish is preferred mine is made by Le Creuset because it produces a delectable crust underneath the potatoes.
A hand slicing device, such as a traditional French stainless steel mandoline or a much less expensive but excellent plastic Japanese made device manufactured by Benriner.
Let the butter soften at room temperature for an hour or so. Meanwhile, preheat your oven to 425°F (220°C).
Place the milk, garlic clove, white pepper, salt, and nutmeg in a small saucepan, stir, bring to a boil, and remove from the heat.
Meanwhile, liberally butter the bottom of the baking dish using about half the butter. Peel the potatoes, rinse them, and pat them dry. Then, slice them 1/8 inch thick, discarding the smallest slices. (This is easier with a slicing machine, inexpensive or elaborate. The quantities and cooking times given here work out best when the slices are even and close to 1/8 inch. Just keep adjusting your slicing machine until a little pile of eight slices measures an inch high.) Under no circumstances should you wash the potatoes after they have been sliced the surface starch is absolutely indispensable.
Evenly arrange the potatoes in the buttered dish in one layer of overlapping slices. (Begin by laying out a row of slices along one narrow end of the baking dish, overlapping each one about a third of the way over the slice that came before. Repeat with a second row, overlapping the entire row about a third of the way over the first row. Continue until the baking dish is neatly paved.) You will undoubtedly have some slices left over. Please do not try to cram them in.
Bring the milk to the boil again and pour it over the potatoes, removing the garlic. Cover the pan with a sheet of aluminum foil. Bake in the middle of the oven for about 15 minutes, until most of the milk has been absorbed. Meanwhile, bring the cream to a boil, and remove from the heat. When the potatoes are ready, remove and discard the aluminum foil. Bring the cream back to the boil and pour it over the potatoes, dotting the surface with the remaining butter.
Bake, uncovered, for another 20 to 25 minutes, until the potatoes have turned a golden brown, spotted with darker, crisp areas. (Rotate the baking dish halfway through if the gratin is browning unevenly.) The underside of the gratin will also be brown and crispy in spots. But do not wait until most of the cream has broken down into clear, foamy butterfat. The potatoes should be dotted with thickened, clotted cream, especially between the slices.
Let the gratin settle for 10 minutes. (This will allow the excess butterfat to drain to the bottom of the dish.) Then eat immediately taste and texture suffer with each passing minute. Cut into 6 or 8 rectangles with a blunt knife and serve each one with a thin, wide metal slotted spatula.