Preheat the oven to 375°F. Butter a 13 × 9-inch baking dish.
Have ready:
After washing, shake off as much water as possible and place cut side down in the baking dish. Sprinkle with:
Pour over the leeks:
Cover the dish with foil and bake until the leeks are tender, about 30 minutes. Remove the foil and sprinkle with:
Return to the oven and bake until browned, about 15 minutes more.