(Nutrition information is calculated using an ingredient database and should be considered an estimate.)
Combine the flour and salt in medium bowl. Cut in plant butter with pastry blender or food processor until coarse crumbs form. Add ice water, 1 tablespoon at a time, until dough forms. Knead dough with floured hands and shape into a disc. Wrap the dough in plastic wrap and refrigerate until chilled, about 30 minutes.
Adjust oven rack to the lowest position and preheat oven to 375°. Unwrap disc of dough and roll on a lightly floured surface from the center to the edges to form a 12-inch circle. Press into 10-inch springform pan.
Line pie dough with aluminum foil or parchment paper, and fill with pie weights. Bake crust until it is dry, about 15 minutes. Let crust cool completely.
Melt plant butter in a large skillet over medium heat. Add the leeks and sauté until golden brown. Remove skillet from heat and allow the leeks to cool slightly.
Preheat oven to 275°.
Mix all ingredients together in a large bowl. Pour into prepared baked pie shell. Bake for 90 minutes, uncovered. Raise oven temperature to 375° and cook for another 10 minutes, or until the top is browned.