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.