Make sure the potatoes are still warm when tossing the dressing, as the spuds will soak up all that citrusy goodness.
In pot, boil potatoes in salted water until tender, 20 minutes; drain. In bowl, stir together butter, preserved lemon (or Meyer lemon zest and juice), and 2 tbsp. dill. Toss warm potatoes with butter mixture, onion, and feta; season. Garnish with pepperoncini and more dill.