Preheat the oven to 160oc (fan oven).
Add the butter into a large soup pan and fry the bacon slices for a few minutes.
Add the onions and cook for 5 minutes. Remove the bacon rashers and set aside in an oven proof dish.
Pop the bacon into a hot oven to crisp up while you continue with the soup.
Add all of the sliced potatoes into the pan with the onions and stir through. Season the potatoes and onions well with the salt and pepper.
Pour in all of the milk and stock. Simmer the soup on a medium heat and be careful not to bring the soup to a boil.
Place 3/4 of the soup into a blender and blend until smooth (be very careful with hot liquids in the blender). Mash the remaining soup in the pan to create texture. You can blend it all if you wish.
Remove the bacon from the oven and set aside.
Add all of the blended soup back into the mashed soup and stir through.
Chop the crispy bacon pieces and add back into the soup. Leave some to sprinkle on the top of the soup.