2 medium leeks, white and tender green parts only, halved lengthwise and
6 cups water
3 tablespoons unsalted butter
1 large sweet onion, such as Vidalia, minced
1 1/2 cups medium-grain Italian rice, such as Arborio (10 ounces)
1/2 cup dry white wine
1/2 cup freshly grated Parmesan cheese
Freshly ground pepper
2 tablespoons snipped chives
Re-form the leeks and tie them together with kitchen twine. In a medium saucepan, bring the water to a boil and add salt. Add the leeks, cover and cook until tender, about 8 minutes. Transfer to a plate to cool slightly; reserve the leek broth and keep warm. Untie the leeks and cut into thin strips.
Melt the butter in a large saucepan. Add the onion and cook over low heat, stirring occasionally, until softened, about 7 minutes. Add the rice and stir until evenly coated with butter, about 2 minutes. Add the wine and simmer over moderate heat, stirring, until almost evaporated, about 3 minutes.
Add 1 cup of the reserved leek broth to the rice and cook, stirring constantly, until the liquid has been absorbed by the rice. Continue to add 4 more cups of the leek broth to the rice, 1 cup at a time, stirring, until the liquid has been absorbed by the rice, before adding more. When the rice is almost tender, after about 16 minutes, remove it from the heat and cover for up to 1 hour.
To reheat, bring the remaining 1 cup of leek broth to a boil. Bring the risotto to a bare simmer over moderately high heat. Add the leeks and stir the risotto with a large spatula until smooth. Add the hot leek broth and stir until the risotto is creamy, about 3 minutes. Stir in the Parmesan cheese and season with salt and pepper. Garnish the risotto with the chives and serve.