Preheat oven to 350º
2 tablespoons butter, unsalted
2 large leeks, trimmed and halved lengthwise
1 1/2 pounds potatoes, Yukon Gold, sliced 1/8″ thick
1 teaspoons kosher salt
1/2 teaspoon black pepper, ground
2 sprigs thyme
1 cup heavy cream
1-2 cloves of garlic, finely chopped
1 bay leaf
1/4 teaspoon nutmeg, ground
3/4 cup cheese, Gruyere or Swiss, grated
Butter 2-quart gratin dish or casserole. Wash leeks to remove any grit and slice thinly crosswise.
Slice potatoes using a mandolin or sharp knife into 1/8″ rounds. Toss with 2/4 teaspoon salt and 1/4 teaspoon pepper. Layer the rounds in the casserole
Melt 2 tablespoons butter in a large skillet over medium heat. Add leeks, thyme, remaining salt and pepper. Cook 5-7 minutes, stirring until leeks are tender and golden. Discard thyme. Scatter leeks over potatoes in casserole.
Add to skillet the cream, garlic, bay leaf, scraping up browned bits of the leeks from the pan bottom. Simmer gently 5 minutes. Stir in nutmeg
Pour cream mixture over leeks and potatoes. Top with the cheese. Cover with aluminum foil and bake 40 minutes.
Uncover and bake until cheese is bubbling and golden, 15-20 minutes longer.