Heat oven to 375°. Heat oil in a large skillet, add leeks and stir occasionally until soft, 3–4 minutes. Add squash and cook until tender, about 5 minutes. Stir in potatoes, chopped dill, feta, salt, pepper and toss to coat everything nicely, letting the cheese melt or soften slightly.
While squash is cooking, roll out piecrust onto lightly floured surface. Spoon squash mixture into the middle of the crust, spreading out evenly, about 1 inch away from the edges.
Gently fold over piecrust edges, each fold over the other, all pointing toward the center, to create a rustic tart edging with exposed vegetables in the middle. Repeat for second tart.
Transfer both tarts to parchment-lined baking sheets and bake for 40–50 minutes, or until crust bubbles and lightly browns.
Remove from oven, sprinkle with dill and allow to rest 5 minutes before serving.