Preheating the pan helps the leeks take on some color; cooking at a lower temperature ensures they're fully tender.
- 6 leeks, white and pale-green parts only
Place a rimmed baking sheet in oven and preheat to 400°. Cut leeks in half lengthwise. Rinse well and pat completely dry. Toss with oil in a large bowl; season with salt.
Arrange leeks, cut side down, on hot baking sheet and cover loosely with foil. Reduce oven temperature to 300°. Bake until leeks are lightly browned on cut side and very tender, about 1½ hours.
Uncover leeks and turn cut side up. Increase oven temperature to 400°; roast leeks until golden brown, 15–20 minutes. (Reserve the oil for making vinaigrettes or roasting vegetables. Let cool; cover and chill.)
DO AHEAD: Leeks can be baked 4 hours ahead. Let cool; cover and chill. Bring to room temperature before serving.