I think this is a Martha Stewart recipe.
2 Tb butter
2 leeks, trimmed, thinly sliced, rinsed well, about 1 C
6 russet potatoes (2.5 pounds) peeled and thinly sliced
8 oz Gruyere cheese, shredded
1 C heavy cream
1 C chicken stock
Oven to 350. Oil a 12-C baking dish. Melt butter in a skillet over medium heat. Add leeks and cook until translucent, 3-4 minutes.
Arrange 1/3 potatoes in the dish, overlapping slices a little. Salt and pepper. Then 1/2 the leeks and 1/3 cheese. Repeat. Top with remaining potatoes and sprinkle with remaining cheese.
Combine cream and stock, pour over cheese and potatoes. Cover with parchment and foil. (Mixture can be refrigerated overnight.)
Bake for 30 minutes. Increase temperature to 425, uncover, cook until top is golden and potatoes are done,a bout 45 minutes. Rest 15-30 before serving.