Using a large pan place on a medium heat, drizzle with oil and add butter. Throw in the leeks to gently soften. I think the best way to make the soup full of flavour is to be patient allowing them to soften slow and go golden. Whilst they are softening, chop the onion and garlic and then throw in the pan stirring well.
Meanwhile stirring regularly, chop up the potatoes.

After about 15 minutes the leeks and onions should be reduced and sweated down.  Next add the stock to the mixture, add the milk and potatoes.  Bring up to boil and a gentle simmer.