Begin by washing the potatoes thoroughly under running water to remove any dirt. Place the cleaned potatoes into a large pot filled with salted water, ensuring they are fully submerged. Bring the pot to a boil over high heat, then reduce to a simmer. Cook the potatoes for 15-20 minutes, or until they are fork-tender but not falling apart.

Once the potatoes are cooked, carefully drain them and allow them to cool for a few minutes. Place each potato on a lined baking sheet and gently press down to smash them to about half an inch thickness.

Preheat your oven to 425°F (220°C). Drizzle olive oil over the smashed potatoes, then season with garlic powder, kosher salt, and black pepper. Toss lightly and roast in the oven for 25-30 minutes, flipping halfway through.

While the potatoes are roasting, combine mayonnaise, Dijon mustard, chopped herbs, and green onions in a medium bowl. Stir well and adjust seasoning to taste.

Once the potatoes are crispy, remove them from the oven and let them cool slightly. In a large mixing bowl, toss the warm smashed potatoes with the prepared dressing until well coated. Chill for 30 minutes before serving.