Preheat your oven to 425°F (220°C). Line a large baking sheet with parchment paper.

Place the potatoes in a large pot and cover with cold, salted water. Bring to a boil and cook until fork-tender but not falling apart (about 15-20 minutes). Drain well and let them cool slightly (about 10 minutes).

Place the slightly cooled potatoes on the prepared baking sheet. Using the bottom of a sturdy glass or potato masher, gently press down on each potato until it is flattened to about 1/2 inch thick. Drizzle generously with olive oil and sprinkle with coarse salt and black pepper.

Roast for 25-35 minutes, flipping halfway through, until the edges are deeply golden brown and crispy. Remove from the oven and allow to cool for at least 15 minutes before handling extensively.

While potatoes cool, prepare the dressing: In a medium bowl, whisk together the mayonnaise, Dijon mustard, apple cider vinegar, chopped fresh dill, diced celery, minced red onion, and garlic powder. Season with salt and pepper to taste.

Once the potatoes are cool enough to handle without breaking, gently transfer them to the bowl with the dressing. Fold gently until the potatoes are thoroughly coated. Be careful not to crush them completely; you want texture.

For best results, chill the potato salad for at least 1 hour before serving to allow the flavors to meld. Garnish with extra fresh dill before serving.