Heat oven to 425°F (220°C). In a large pot, cover the potatoes, 1 tbsp kosher salt, and baking soda with cold water by 2 inches. Bring to a boil, then simmer until the potatoes are just fork-tender but not falling apart, 8-10 minutes. Drain well and let them sit in the colander for 1 minute to steam off moisture.

Return the drained potatoes to the dry pot, cover, and shake firmly until the surfaces are scuffed and floury. Add the olive oil and a generous grind of black pepper, then toss gently to coat each piece in the starchy glaze.

Transfer the potatoes to a rimmed baking sheet, spreading them into a single layer with space between each chunk. Roast on the middle rack, undisturbed, for 25 minutes.

Flip the potatoes with a thin spatula until the golden undersides face up. Roast for another 15-20 minutes, until deeply browned and sizzling. Pile onto a warm dish, sprinkle with flaky sea salt, and serve immediately.