Boil the potatoes. Put the cubed potatoes into a large stock pot filled with cold water, add 1 tablespoon salt, and bring it to a boil over high heat. Once it’s boiling, drop the heat to medium-high and cook the potatoes until tender, about 12 to 16 minutes. They’re ready when a fork slides into a piece without resistance. Drain them.

Heat the oven and season the potatoes. Preheat the oven to 400°F and line a baking sheet with aluminum foil, then set it aside. In a bowl, combine the drained potatoes with the olive oil, garlic powder, salt, and pepper, tossing gently so the edges stay intact and every piece gets an even coating.

Roast until deeply browned. Spread the potatoes across the prepared baking sheet in a single even layer, giving them room rather than piling them up. Bake for about 30 minutes, until they’re very brown and crispy on the outside.

Start the dressing base. While the potatoes roast, combine the sour cream and mayonnaise in a large bowl and stir until the mixture is smooth and one uniform color.

Finish the dressing. Reserve 1 tablespoon of the scallions for later, then add the remaining scallions to the bowl along with 1 tablespoon of the dill, 1 tablespoon of the parsley, the dijon mustard, and the lemon juice. Stir to combine, then season with salt and pepper to taste. Taste after the acid is in, since lemon and mustard both sharpen how salty it reads.

Dress at the last moment. Wait until right before serving to toss the potatoes in the dressing. Dressing them any earlier lets moisture soak into the crisp surfaces and softens them.

Garnish and serve. Transfer everything to a serving dish and top with the reserved scallions along with the remaining dill and parsley.