Place potatoes in a large pot, cover with cold water by 1 inch, and add a generous handful of kosher salt. Bring to a boil, then simmer until fork-tender (15-20 minutes).

Drain the potatoes and return them to the hot, empty pot off the heat for 3-5 minutes to allow excess surface moisture to steam off.

Preheat your oven to 425°F (220°C). Generously grease a large, heavy rimmed baking sheet with 1 tablespoon of olive oil.

Transfer the dry potatoes to the baking sheet. Using the flat bottom of a heavy glass or a potato masher, gently but firmly press down on each potato to smash it to a 1/2-inch thickness.

Drizzle the smashed potatoes with the remaining 2 tablespoons of olive oil and season generously with salt and pepper. Roast for 25-30 minutes until bottoms are deeply golden, then flip and roast for another 10-15 minutes until fully crisp.

Remove from the oven and let the potatoes cool on the pan for 15-20 minutes. They should be warm, but not piping hot, or they will melt the dressing.

While potatoes cool, whisk together the mayonnaise, sour cream, Dijon mustard, lemon juice, celery, red onion, dill, and chives in a large mixing bowl until smooth.

Gently chop the cooled, crispy potatoes into large, bite-sized pieces. Add them to the bowl with the dressing and gently fold until just combined. Serve immediately.