Luxuriously silky leeks--slow-cooked in butter and seasoned with lemon juice and thyme--ripple through sweet potatoes. Feel free to substitute ordinary potatoes if you prefer.
rim roots and wilted leaves from leeks. Cut in half lengthwise. Slice each half crosswise into 1/4-inch slices. Place in a colander and rinse well to remove any grit. Drain and dry leeks.
In a 5- to 6-quart Dutch oven, cook sweet potatoes with 1 teaspoon of salt in enough boiling water to cover for about 10 to 12 minutes until potatoes are just tender. Drain; return to pot.
Meanwhile, heat butter in a large skillet over medium-high heat until melted and beginning to bubble. Add leeks, reduce heat to medium and cook for about 6 minutes, or until leeks are just tender, stirring occasionally. Add stock, lemon zest and juice, thyme, pepper and remaining 1/2 teaspoon salt. Cook and stir for 3 minutes.
Add about two-thirds of leek mixture to potatoes and mash with a potato masher until desired consistency. Transfer to a serving bowl and top with remaining leek mixture.
Cut up sweet potatoes can be held submerged in water in the refrigerator overnight. Drain and use as directed. Leeks can be prepared ahead and refrigerated for up to 4 hours. When ready to use heat until just warmed through.