Place potatoes in a large pot and cover with water. Add 1 tablespoon salt and bring to a boil over high heat. Reduce heat to medium-high; cook until potatoes are tender when pierced with a fork, 12 to 15 minutes; drain.
Preheat the oven to 425 degrees F (220 degrees C). Line a rimmed baking sheet with parchment or foil.
Coat the prepared baking sheet with 1 tablespoon olive oil. Add potatoes in a single layer onto the pan. Using a heavy-bottomed jar or glass, smash potatoes to 1/4 to 1/2 inch thickness, taking care not to fully separate the potatoes. Drizzle smashed potatoes evenly with remaining oil; sprinkle with remaining teaspoon salt, 1/2 teaspoon pepper and garlic powder.
Roast potatoes in the preheated oven, until very crispy and browned, about 30 minutes.
While potatoes are roasting, prepare the dressing. Combine the remaining 1/4 teaspoon of pepper with sour cream, mayonnaise, mustard, lemon juice, dill, and parsley. Reserve 1 tablespoon of green onion for garnish; add the remaining green onions to the sour cream mixture. Stir well to combine. Taste for seasoning and adjust as desired.
Reserve 1/4 cup crispy potatoes for topping and stir remaining potatoes with dressing until well coated. Add to a serving dish and top with reserved green onion, crispy potatoes, and more dill and parsley as desired.