Preheat your oven to 200 degrees celsius.
Line 2 baking trays with greaseproof paper and set aside.
Put a medium pan of water on the stove with ½ tsp salt and a lid. Bring to the boil.
Peel and grate your potatoes. Add to the boiling water and blanch for 1 minute. Drain thoroughly in a colander. Transfer the potatoes to one of the baking trays and pop in the hot oven to dry out for 2 minutes. Remove from the oven and set aside to cool.
Remove your puff pastry sheets from the freezer and set aside.
Put your spring onion, brown onion and garlic into a small food processor or blender and blitz until finely chopped. Add a generous amount of salt and pepper.
Transfer your cooled potatoes to a large bowl, add the onion mixture, cheddar and 1 tsp salt. Stir thoroughly to combine.
Crack the egg into a small bowl and whisk with a fork to combine.
Take your puff pastry sheets and cut each sheet into four equal squares. Lay four squares on each baking sheet, making sure that they’re evenly spaced.
Weigh your potato, onion and cheese mixture and divide by 8.
Using a spoon, put an eighth of the mixture into the centre of each pastry piece. Dip your clean finger or a pastry brush in the egg and paint two edges of the square with the egg. Fold the pastry over into a triangle shape, line up the edges of the pastry and seal using the prongs of a fork. Cover the surface of the tartlet with the egg wash and slice three 1 inch long lines across the top of the tart with a sharp knife. Continue this process with all remaining tartlets.
Pop into the oven and bake for 18 minutes. Check the tartlets after this time, you want the pastry to be a deep golden brown all over. If it’s not quite ready, pop back into the oven for 4-7 minutes depending on how much longer you think they need.
Serve warm with a side salad and they’ll make an excellent entree.