Preheat oven to 230°C.
Pour oil over baking sheet and tilt until evenly coated with oil.
Halve potatoes lengthwise. Place potato halves cut side down on cutting board and cut lengthwise into 2 cm thick wedges (3 to 4 wedges per half potato, depending on size).
In a large pot, bring water to a boil over high heat. Add salt, baking soda, and potatoes and stir to combine. Return to a boil, then reduce heat to a simmer, and cook until potatoes are barely tender, offering some resistance when poked with a paring knife or cake tester (the texture should be similar to that of a firm poached pear or lightly cooked apple), about 7 minutes from when potatoes are added to the pot. Drain potatoes, taking care not to break the wedges, and transfer to a large bowl.
Season potatoes with salt and pepper and toss to coat, shaking bowl roughly, until a thick layer of mashed potato–like paste has built up on the surface of the wedges.
Transfer wedges to prepared baking sheet, arranging them in a single layer with one flat cut side down, and space between each piece.
Transfer to oven and roast, without moving, until potatoes are golden brown on bottom side, 18 to 22 minutes. Flip potatoes onto second flat cut side and continue to roast until golden brown on second side, 18 to 22 minutes longer.