Heat the oven to 220C gas 7.
Place the potatoes into a pan and cover with cold water.
Bring to the boil and then simmer until really well cooked, just about falling apart, this is really important, then drain well.
Heat the oil and add the potatoes once nice and sizzling pop in the oven and cook until really really crisp and fried. The crisper the better. In fact overcook them. This is why you need to overcook so you get lots of crunch and flavour.
You can actually fry them until crisp if you want to.
Meanwhile, heat 2 tbsp oil and cook the onions, leeks and garlic for 15 minutes to take a little colour.
Once the potatoes are ready add to the leeks and onions and really mix well.
Cover with water and add the stock cubes and bring to the boil.
Once boiling add the chestnuts and simmer for say 15 minutes until all the vegetables are cooked.
Once cooked, gently break up with as masher but not too much.
Re season add the parsley and serve.