1. Fill a large pot fitted with a steamer basket with enough water to come just below basket; bring to a boil. Add potatoes; reduce to a simmer, cover, and cook until fork-tender, about 20 minutes.

2. Preheat oven to 425 degrees. Divide potatoes between 2 rimmed baking sheets; lightly mash using a metal spatula. Drizzle with oil. Season with 1/2 teaspoon coarse salt, pepper to taste, and thyme. Slide potatoes around on sheet to coat bottoms with oil.

3. Roast, rotating sheets once, until skins are golden and crisp, about 25 minutes. Transfer to a platter. Drizzle with vinegar, sprinkle with flaky salt, and serve.