Wash and scrub the potatoes. Preheat the oven to 200 degC. Cut a thin slice from the base of each potato so that they will sit flat on a cutting board. Place a potato on the cutting board and lay a wooden spoon alongside the length of it to stop you from cutting right through to the bottom of the potato. Using a sharp knife, held at right angles to the wooden spoon, thinly slice each potato but do not cut all the way through.
Make cups from 4 pieces of tinfoil and place slices of garlic and onion in each one. Place a potato in each 'cup' and place into a baking tin. Using a pastry brush, brush vegetable oil over the top of each potato and use the brush to carefully push some of the oil between the slices.
Bake for around 45 minutes.
While the potatoes are cooking, combine the ingredients for the spice topping in a small bowl and mix well. After 45 minutes, brush the mixture over the potatoes and into the slices. Return potatoes to the oven and bake for a further 15 minutes or until brown and crispy. Garnish with the coriander.