Put the cubed potatoes in a 3-quart / 2.8 L saucepan, cover with cold salted water by 2.5 cm / 1 inch, and bring to a boil. Cook for 12 to 15 minutes until a knife slides in with no resistance.

Drain well and return the potatoes to the hot pot for 1 minute over low heat, shaking the pot, to steam off extra moisture. Mash with a fork or potato masher until mostly smooth, then spread on a plate to cool for 10 minutes.

Heat a 30 cm / 12-inch cast-iron or nonstick skillet over high heat with no oil. Add the whole trimmed spring onions and char for 4 to 5 minutes, turning occasionally, until blistered and black in spots and the white parts have softened.

Transfer the onions to a board, let cool for a couple of minutes, then chop finely. Set aside 2 tbsp of the darkest, most charred bits for garnish.

In a mixing bowl, combine the cooled mashed potato, chopped charred onions, egg, flour, parmesan, garlic, salt, and pepper. Mix with a fork just until combined. If the mixture feels wet and won't hold a shape, add another tablespoon of flour.

Divide the mixture into 8 portions, about 130 g each, and shape into patties roughly 2 cm / 3/4 inch thick. Dust lightly with flour on both sides.

Set the patties on a sheet pan and chill for 15 minutes. This firms them up so they hold together in the skillet.

Wipe out the skillet and set over medium heat with 2 tbsp olive oil. Once the oil shimmers, add 4 patties, leaving space between them, and fry for 4 to 5 minutes per side until deep golden and crisp at the edges.

Transfer to a paper towel-lined plate. Add the remaining 2 tbsp oil to the skillet and fry the second batch of 4 patties the same way.

Serve warm topped with a spoonful of sour cream or yogurt, the reserved charred onion bits, and a scatter of chives.