Place the sliced red onion in a bowl with the vinegar, sugar, a pinch of salt, and 2 tablespoons of boiling water. Set aside to pickle while preparing the rest of the salad.

Add the potatoes to a large pot of salted water. Bring to a boil and cook until fork-tender, about 15–20 minutes.

Preheat the oven to 425°F.

Drain the potatoes and spread them on a baking sheet. Gently press each potato with the bottom of a glass or spatula until slightly flattened.

Drizzle the potatoes with olive oil and season with salt and pepper.

Roast for 20–25 minutes until crisp and golden at the edges.

Drain the pickled onions, reserving the pickling liquid.

In a small bowl, whisk the reserved pickling liquid with the Dijon mustard, olive oil, and most of the chopped dill to make the dressing.

Transfer the roasted potatoes to a serving platter. Sprinkle with feta, drizzle with dressing, then scatter the pickled onions and remaining dill over the top.

Serve warm or at room temperature.