Preheat oven to 350˚F. Generously butter a 9x14-inch baking dish.

In a large frying pan, melt the butter over medium heat. Add the leeks and cook, stirring occasionally until tender, about 7 minutes. Remove from heat.

Mix together the salt and pepper. Peel the potatoes and thinly slice into rounds.

Spread one-third of the potatoes in an even layer in the prepared baking dish and season with 1/3 of the salt and pepper. Top with one-third of the Gruyère and half of the leeks.

Top with half of the remaining potato slices, half the remaining Gruyère, and the rest of the leeks; sprinkle with half the remaining salt and pepper. Finish with the remaining potatoes, Gruyère, and salt and pepper.

In a small saucepan, heat the cream over medium-high heat until it is simmering. Pour the hot cream evenly over the potatoes. Cover tightly with aluminum foil and place the baking dish on a rimmed baking sheet. 

Bake for 1 hour. Remove the foil and continue baking until the potatoes are tender, coated with a creamy sauce, and golden brown on top, about 30 minutes longer.

Let stand for about 5 minutes, then serve hot.