Bring the water to a boil in a large pot. One it reaches boiling point, remove it from the heat and add in the vinegar and a tablespoon of sea salt. Stir until the salt dissolves and then place the potatoes in the pot. Let the potatoes soak in the vinegar mixture for an hour.
Preheat the oven to 450 degrees F. Drain the potatoes and pat them dry with a kitchen towel. Toss the potatoes with a tablespoon of the oil, the cornstarch, and a teaspoon of salt. Toss well, and then drizzle the remaining tablespoon of oil on a walled baking sheet. Put the baking sheet (without the potatoes on it!) in the oven to heat up the oil for 3 minutes.
Spread the potatoes on the baking sheet, sprinkle with a bit of rosemary, and place back in the oven for 35-40 minutes, flipping the potatoes and rotating the baking sheet half-way through.
Once the potatoes are crisp, remove them from the oven, sprinkle with additional sea salt, and drizzle with a bit more vinegar. Garnish with fresh rosemary and serve warm.