Roughly chop mushrooms and slice the leeks. Heat 3 Tbsps of oil in a pan and gently sauté the chopped vegetables for 15 minutes until they're tender.
Add the cornstarch, stir for 1-2 minutes, then pour in the milk and the broth. Bring to the boil, then gently simmer it (over low heat) for half an hour.
Meanwhile, chop the remaining mushrooms and fry them in the remaining oil with the pine nuts.
Blend the soup until smooth and silky, then add the lemon juice and season with salt & pepper to your taste.
Serve with the fried mushroom and pine nuts pieces scattered on the top.