Preheat oven to 450°F (230°C) with rack in center position. If using convection, reduce to 400°F (200°C). Heat 2 quarts water in large pot until boiling.

Add 2 tablespoons kosher salt and baking soda to boiling water. Add potato chunks, return to boil, then simmer until knife meets little resistance, about 10 minutes after returning to boil.

While potatoes cook, combine fat with rosemary, garlic, and black pepper in small saucepan. Heat over medium heat, stirring constantly, until garlic begins to turn golden, about 3 minutes. Strain oil through fine-mesh strainer into large bowl. Reserve garlic-rosemary mixture separately.

Drain potatoes and let rest 30 seconds. Transfer to bowl with infused oil, season with salt and pepper, then toss to coat. Shake bowl roughly until thick paste builds up on chunks.

Transfer potatoes to large rimmed baking sheet, spreading evenly. Roast without moving for 20 minutes.

Using thin spatula, release stuck potatoes and shake pan. Turn potatoes to expose different surfaces. Continue roasting until deep brown and crisp, turning every 15 minutes, for 30-40 minutes longer.

Transfer to large bowl. Add reserved garlic-rosemary mixture and parsley. Toss to coat and adjust seasoning with salt and pepper.