A good gratin should arrive at the table bubbling at the edges, with potatoes tender all the way through and a top that gives a little resistance under the spoon. Leeks bring gentle sweetness here, so the cream needs only Dijon mustard, thyme, and enough salt to make the vegetables taste like themselves.
At a glance
This makes 6 generous servings, with about 25 minutes of preparation and 60 to 65 minutes in the oven. Use a 9-by-13-inch baking dish and slice the potatoes to roughly ⅛ inch so they cook at an even pace.
Ingredients to gather
You will need 2½ pounds Yukon Gold potatoes, peeled; 2 large leeks, white and light-green parts only; 1 garlic clove, halved; 1 cup heavy cream; 1½ cups whole milk; 1 cup vegetable or chicken broth; 1 tablespoon Dijon mustard; 1 teaspoon fresh thyme leaves; 1½ teaspoons kosher salt; ½ teaspoon black pepper; and 1 cup grated Gruyère.
Layer and bake
Step 1: Heat the oven to 375°F and rub the baking dish with the cut side of the garlic. Slice the leeks lengthwise, rinse between their layers to remove grit, and cut them crosswise into thin half-moons; slice the potatoes evenly. Step 2: Warm the cream, milk, broth, mustard, thyme, salt, and pepper in a saucepan until steaming, then take it off the heat.
Step 3: Arrange half the potatoes in the dish, scatter over half the leeks, and repeat with the remaining vegetables. Pour the warm cream mixture over the layers, cover tightly with foil, and bake for 40 minutes. Step 4: Remove the foil, scatter the Gruyère over the top, and bake for another 20 to 25 minutes, until browned and a thin knife slides easily through the center.
Let the layers settle
Give the gratin 10 minutes on the counter before serving; the sauce will thicken slightly and the first spoonful will hold together better. If the top is golden before the potatoes are tender, cover the dish loosely with foil and keep baking until the knife test says they are ready.