A mandoline will slice the potatoes quickly and to the same thickness, though a sharp knife will also work. Instead of being buried in cream, the potatoes and leeks are simmered in and drizzled with milk so the potatoes get wonderfully crisp and tender and the cheeses form a melty, golden crust.
How to Make It
Preheat oven to 375°F.
Combine milk, potatoes, and garlic in a large saucepan; bring to a boil. Reduce heat and simmer 5 minutes or until potatoes are almost tender. Drain in a colander over a bowl, reserving 1/2 cup milk mixture.
Melt butter in a 10-inch cast-iron skillet over medium heat. Add leek; cook 8 minutes or until tender, stirring occasionally. Remove pan from heat; place leek mixture in a bowl. Arrange half of potato slices in a single layer in bottom of skillet or a gratin dish; top evenly with half of leek mixture. Sprinkle with 1/2 teaspoon salt and 1/4 cup Gruyère cheese. Repeat layers with remaining half of potatoes, remaining half of leek mixture, remaining 1/2 teaspoon salt, and remaining 1/4 cup Gruyère. Drizzle with reserved 1/2 cup milk mixture; sprinkle with Parmesan. Cover pan with foil; bake at 375°F for 25 minutes or until cheese melts. Remove foil from pan. Turn on broiler (do not remove pan from oven). Broil 5 minutes or until bubbly and golden brown. Remove pan from oven, and sprinkle with pepper; cool. Cut into 10 wedges.