prepare the vegetables
Cut the leek lengthways and clean under running water. Cut into small chunks. Peel the potato and cut it into 1 cm x 1 cm (1/2-inch x 1/2-inch) cubes.
cook the soup
Place a pot over medium heat. Add butter and olive oil. When the butter melts, add the leek and potato. While regularly stirring, cook for 5 minutes. Pour in the water or vegetable broth. Bring to a boil, then simmer gently, covered with a lid, for 12 - 14 minutes or until the potato is cooked.
blend and serve
Transfer the soup to a blender and mix until you get a smooth and creamy soup. Feel free to use an immersion blender. Season with salt and pepper to taste, then stir in the heavy cream (or whipping cream). Divide the soup between four plates. Optionally serve with crunchy croutons.