In a large pot, fry the bacon until crispy (about 5 minutes) and remove from the pot; set aside
Add the butter into the bacon fat and continue to cook until melted (I never said this was low fat!).
Add the leeks and onions and cook until softened and slightly browned.
Add the broth, potatoes and oregano and cook until the potatoes are tender (about 15 minutes); add the greens and warm through.
Once the potatoes are soft, use an immersion blender to make it smooth (I like the soup to have a few lumps because it feels more rustic to me). Alternatively, you can use a blender; just use the pulse button so you don't make it completely smooth.
Pour the soup into some nice bowls; top with crumbled cheese, the bacon and some chives to make it look pretty and some delicious oil if you have some!