Place all ingredients, but only half of the leeks (reserving the other half for topping), into a slow cooker. Set cooker for 8 hours. Go to work. Come home and smell the goodness! Give soup a good stir and add more broth, salt and pepper, if necessary. In a pan, add butter and remaining leeks. Sauté leeks until softened. Add a drizzle of honey and cook further until a golden color.
Ladle soup into bowls and top with the leeks.