Preheat the oven to 180°C (160°C fan). Place a baking dish in the oven to heat.
Place the whole peeled potatoes in a large saucepan. Cover with water, add coarse salt and bicarbonate of soda, and boil for about 20 minutes, until half cooked.
Drain carefully, then leave the potatoes in a colander for 5 minutes to steam-dry.
Toast the ground coriander in a dry pan for about 30 seconds. Melt the butter, then whisk in the oil, brown onion soup powder, coriander, black pepper, smoked paprika, garlic, Worcestershire sauce and brown sugar. Stir in 50 g of the biltong; reserve the rest for finishing.
Score each potato at 1 cm intervals, cutting only about two-thirds of the way down. Rest each potato between two wooden spoons to avoid cutting all the way through.
Add a few spoonfuls of the biltong butter to the hot baking dish. Arrange the potatoes in the dish and baste generously, working the butter between the slices.
Roast for approximately 1 hour, basting after 25 minutes and again after 45 minutes, until the potatoes are deep golden and crisp.
Let the potatoes rest for 3 minutes, then scatter over the reserved biltong, parsley and lemon zest. Finish with flaky sea salt and serve warm.