Crispy Sea Salt and Vinegar Roasted Potatoes

Bahan-bahan (4 porsi)

• 1 ½ lbs fingerling potatoes, cut in half
• 4 cups water
• 2 cups white vinegar, plus more for drizzling
• Sea salt, to taste
• 2 tablespoons oil, divided
• 1/2 tablespoon cornstarch
• Rosemary, to taste (optional)