Heat the oil and butter in a large saucepan, add the bacon, white leeks and diced potatoes and cook over a low heat for 5 minutes, stirring until lightly golden.
Mix in the stock, then bring to the boil, cover and simmer for 20 minutes until the potatoes are just tender. Add the chicken and boil rapidly for 3 minutes.
Stir in the green leeks, milk, cream and a little salt and pepper. Simmer gently for 5 minutes, then stir in the spinach and a little nutmeg. Cook for 2 minutes until the spinach is just cooked.
Ladle into bowls, sprinkle with a little extra nutmeg and serve with crusty bread.