1. Trim roots and dark green leaves from leeks. Halve leeks lengthwise, slice into 1/4"-thick half-moons, and transfer to a large bowl filled with cold water. Swish leeks with your fingers to remove dirt, then set in a colander to drain. Melt 1 tablespoon butter in a large skillet over medium-high heat. Add leeks and 1/2 teaspoon salt and cook, stirring occasionally, until leeks are tender but not browned, about 10 minutes. Let cool 10 minutes, then transfer to a cutting board and finely chop; set aside.
2. Place a rack in center of oven and heat to 375 degrees. Melt remaining 3 tablespoons butter. Brush a 9" springform pan all over with 1 tablespoon melted butter and place on a foil-lined rimmed baking sheet.
3. In a large bowl, combine potatoes, leeks, half of cheese, remaining 2 tablespoons melted butter, cornstarch, thyme, 2 teaspoons salt, and 1/4 teaspoon pepper and gently toss until well-combined.
4. Starting on the outside edge of prepared pan, arrange potatoes in concentric circles, slightly overlapping slices by about 1/4". Continue arranging potatoes, sprinkling leeks and cheese between each layer and tossing potato mixture remaining in bowl occasionally to ensure ingredients are evenly dispersed. Cover finished pan tightly with greased foil (greased side down) and bake until potatoes are just tender, about 1 1/2 hours.
5. Remove pan from oven and carefully remove cover. Sprinkle remaining half of cheese evenly over tart. Run a paring knife between tart and springform band and carefully remove sides of pan. Return pan to oven and bake until potatoes are completely tender and edges are brown, 50 to 55 more minutes; let cool 10 minutes before serving.
Source: Hannaford fresh Magazine, December 2018