For the potatoes:
1) Whisk together the chilli powder, cinnamon, cumin, sugar, salt and pepper, to taste, in a small bowl. Heat the grill over a medium-high heat. Brush the potato slices with oil on both sides and season with the spice rub on one side.
2) Skewer 2 slices of the potatoes onto 2 skewers (so the potatoes lay flat) and place on the grill, rub side down; cook until golden brown and a crust has formed, 2 to 3 minutes. Turn the skewers over and continue grilling until the second side is golden brown and the potatoes are just cooked through, 2 to 3 minutes longer. Remove to a platter and immediately drizzle with spring onion vinaigrette, spring onions and chopped coriander.