Adjust oven rack to center position and preheat oven to 450°F (230°C) (or 400°F (200°C) if using convection). Heat 2 quarts (2L) water in a large pot over high heat until boiling. Add 2 tablespoons kosher salt (about 1 ounce; 25g), baking soda, and potatoes and stir. Return to a boil, reduce to a simmer, and cook until a knife meets little resistance when inserted into a potato chunk, about 10 minutes after returning to a boil.

Meanwhile, combine olive oil, duck fat, or beef fat with rosemary, garlic, and a few grinds of black pepper in a small saucepan and heat over medium heat. Cook, stirring and shaking pan constantly, until garlic just begins to turn golden, about 3 minutes. Immediately strain oil through a fine-mesh strainer set in a large bowl. Set garlic/rosemary mixture aside and reserve separately.

Transfer potatoes to a large rimmed baking sheet and separate them, spreading them out evenly. Transfer to oven and roast, without moving, for 20 minutes. Using a thin, flexible metal spatula to release any stuck potatoes, shake pan and turn potatoes. Continue roasting until potatoes are deep brown and crisp all over, turning and shaking them a few times during cooking, 30 to 40 minutes longer.

Transfer potatoes to a large bowl and add garlic/rosemary mixture and minced parsley. Toss to coat and season with more salt and pepper to taste. Serve immediately.