Preheat oven to 375°. Bring a large pot of salted water to a boil. Add leeks to the pot and simmer until they just start to soften, 4–5 minutes. Use a slotted spoon to transfer leeks to a baking dish. Reserve blanching water for another use. Drizzle leeks with olive oil. Place leeks in oven and roast until soft and the outer layers are slightly browned, 20–30 minutes. Slice the leeks in half lengthwise and serve warm.