Preheat oven to 450°F. Peel and cut the potatoes into large chunks.
In a large pot, cover potatoes with water by 2 inches. Add baking soda and 2 tablespoon salt. Bring to a boil, then lower to a simmer. Cook until mostly tender (~10 minutes).
Meanwhile, in a small sauce pot, combine olive oil with chopped rosemary and garlic. Simmer until garlic is just golden. Strain oil into a heatproof bowl and set garlic-rosemary mix on a paper towel-lined plate.
Drain potatoes in a colander. Return to the pot for 1 minute to steam-dry.
Transfer to the bowl of infused oil and toss vigorously until exteriors are rough and mashed. Season with salt and pepper.
Spread on a large baking sheet and roast for ~20 minutes. Toss and continue roasting for 30–40 minutes more, tossing occasionally, until deep golden brown.
Toss with reserved garlic-rosemary mix, finish with flaky salt, and serve immediately.