Finely dice the leeks, fennel and onion then add them with the garlic to a large saucepan. Add the oil and butter and over med-low heat sauté until soft. This can take about 15-20 min.
Add the wine and reduce by half.
Add the stock, bring to simmer then add the potatoes, stirring regularly. Small potatoes can be added whole, but cut larger ones in half. The potato is your timer, when soft take out and mash separately. The mash you want is a lumpy one.
Blend the soup mixture in a blender or with a hand blender. Again you want some texture so don't blend to a smooth texture. Add the potato mash back in and bring back to simmer. This time don't walk away from your soup as the potato can catch on the bottom.
Once reheated serve and garnish with parsley.
Notes: you can use stock from a supermarket but don't use stock cubes. Do yourself a favour and use the link below and make your own stock. The other thing is buy yourself a pressure cooker, I make the stock in a pressure cooker and cook it for 40 minutes. Heston's stock link: http://www.sbs.com.au/food/recipes/heston-blumenthals-brown-chicken-stock