Heat the oven to 220C/200C fan/gas 7. Cut the potatoes into chunky chips or wedges. Tip into a pan, top up with cold water and bring to a boil. As soon as you see bubbles, simmer gently for 5 mins. Drain carefully and leave to steam dry.
Meanwhile, heat 4 tbsp of the oil in a shallow-lipped baking tray in the top of the oven. Transfer the parboiled chips to the tray, carefully coat in the hot oil and cook for 20-25 mins until golden and crisp, turning halfway through.
While the chips are cooking, toast the peppercorns in a wok or large frying pan for a few mins, then transfer to a pestle and mortar and crush. Heat the remaining in the pan and stir-fry the onion, peppers, garlic and chilli for 5-8 mins over a medium-high heat until the onions and garlic turn golden brown. Remove from the heat and set aside.
Mix the Chinese five-spice powder, sugar, salt, chilli flakes and ground peppercorns together. Tip the cooked chips into the pan with the veg and return to a medium heat. Toss to combine. Sprinkle in the salt and pepper mixture and the spring onions. Toss everything together for 1-2 mins until evenly coated, then serve straight away with another pinch of sea salt, if you like.