Potato Gratin

Rub a 9- by 12-inch gratin dish with: Butter.
Peel and slice about 1/16 inch thick: 4 large yellow potatoes (about 1 1/2 pounds).
Make a layer of potato slices in the gratin dish, overlapping them slightly, like shingles.
Sprinkle with: Salt and fresh-ground black pepper.
Continue to layer the potato slices, seasoning each layer, until the potatoes are used up.
You should have two or, at the most, three layers.
Carefully pour over the potatoes: 1 cup milk.
The liquid should come up to the bottom of the top layer of potatoes.
Add more if necessary.
Generously dot the top of the potatoes with: 3 tablespoons butter, cut into pieces.
Bake in a 350F oven until browned and bubbling, about 1 hour.
Halfway through the baking, take the gratin dish out of the oven and press the potatoes flat with a metal spatula to keep the top moist.
Return to the oven and keep checking.
The gratin is done when the potatoes are soft and the top is golden brown.
Peel and smash a garlic clove and rub it all over the inside of the gratin dish before buttering it.
Use duck fat instead of butter.
Use heavy cream or a mixture of half-and-half and cream.
Omit the butter.
Substitute celery root, parsnip, or turnip slices for up to half the potatoes.
Add chopped herbs such as thyme, parsley, chives, or chervil between the layers.
Saute mushrooms, sorrel, spinach, or leeks, and layer them between the potato slices.
Sprinkle grated Gruyere or Parmesan cheese on each layer and sprinkle more on top for the last 15 minutes of baking.