1. In a large skillet, melt 2 tablespoons butter over medium heat. Add the leeks and cook, stirring occasionally, until they begin to soften, about 8 minutes. Add 14 cup water to skillet. Reduce heat to medium-low. Cook, covered, about 20 minutes or until the leeks are very soft. If the pan becomes dry, add 1 to 2 tablespoons water as needed. Remove from the heat and reserve.
2. Meanwhile, bring a large saucepan of salted water to a boil. Add the potatoes and cook until tender, 12 to 15 minutes.
3. While the potatoes cook, combine the remaining 2 tablespoons butter and half-and-half in a small bowl and microwave until the butter has melted and the mixture is warm.
4. Drain the potatoes and mash. Add the half-and-half mixture and leeks to the potatoes, and continue to mash until desired consistency. Season with salt and pepper.
5. Transfer the mash to a 2-quart casserole and use a butter knife to make peaks that will brown when cooked.
6. Preheat the oven to 400 degrees F. Bake casserole for 30-40 minutes or until heated through. Turn the oven to broil and cook until just browned on top.