Ingredients
1 1/2 pounds Japanese sweet potatoes, peeled and cut into 1-inch pieces (4 cups)
12 ounces turnips, peeled and cut into 3/4-inch pieces (about 2 cups)
Kosher salt and freshly ground white pepper
4 tablespoons unsalted butter
2 leeks, white and light-green parts only, coarsely chopped, well washed and drained (2 packed cups)
1 teaspoon sesame oil, plus more to taste
Preparation
Combine potatoes and turnips in a pot and cover with water by 2 inches. Bring to a boil; season generously with salt. Boil until vegetables are tender and easily pierced with the tip of a knife, 12 to 15 minutes. Reserve 1 cup cooking water, then drain and return vegetables to pot.
Meanwhile, melt butter in a large skillet over medium heat. Add leeks, season with salt, and cook, stirring a few times, until leeks are tender but not developing any color, 5 to 7 minutes.
Transfer mixture to pot with vegetables, along with 1/3 cup reserved cooking water and sesame oil. Mash to combine; season with salt, white pepper, and more sesame oil, 1/4 teaspoon at a time. Stir in more cooking water, a little at a time, to reach desired consistency.
