Preheat oven to 450°F. In a pot, add potatoes and cover with cold water. Season with salt and bring to a boil over medium-high heat. Cook until tender when pierced with a knife, 10-12 minutes. Let cool slightly, then using the bottom of a measuring cup or glass, smash each potato until ¼-inch thick.
Transfer smashed potatoes to a sheet pan and toss gently with ¼ cup of the olive oil and salt. Roast, flipping halfway through, until golden brown and crisp, 35-40 minutes.
Meanwhile, in a medium bowl, whisk together remaining ingredients with remaining ¼ cup olive oil until smooth. Adjust seasoning with salt.
Transfer potatoes to a bowl and spoon dressing on top, then serve.
