1/4 c. (1/2 stick) unsalted butter
4 c. sliced leeks (white and pale green parts only), about 2 lbs.
Salt and pepper
1 tbsp. butter
2 c. whipping cream
3 lg. garlic cloves, minced
4 lbs. russet potatoes, peeled, thinly sliced
2 c. grated white Cheddar cheese or Gruyere cheese (about 8 oz.)
1/3 c. freshly grated Parmesan cheese

Melt 1/4 cup butter in heavy large skillet over medium-high heat. Add leeks and stir to coat. Reduce heat to medium-low. Cover and cook until leeks are tender, stirring occasionally, about 8 minutes. Uncover and cook until almost all liquid is absorbed, about 3 minutes. Season leeks with salt and pepper.

Preheat oven to 375 F. Rub 9x13x2 inch baking dish with 1 tablespoon butter. Mix cream and garlic in small bowl. Arrange half of potatoes in prepared dish. Season generously with salt and pepper. Cover with leeks. Sprinkle with half of Cheddar cheese. Ladle half of cream mixture over. Repeat layering with remaining potatoes, Cheddar cheese and cream mixture. Sprinkle with Parmesan. (Can be prepared 6 hours ahead. Cover and refrigerate. Let stand 1 hour at room temperature before baking.) Bake until potatoes are tender and top is deep golden brown, about 1 hour 15 minutes. Let stand 15 minutes at room temperature before serving.