If your slow cooker has a sauté function, melt the butter directly in the pot. Or, do this in a frying pan over a medium heat. Cook the onion, leeks and bay leaf with a pinch each of salt and black pepper for 10-15 mins, stirring occasionally until all the veg is soft. If you’ve used a frying pan, transfer the softened veg and bay leaf to the slow cooker.

Add the potato and stock to the slow cooker, stir, then cover and cook on low for 6-7 hrs or on high for 3-4 hrs until the potato is completely soft. Remove the bay leaf, then spoon in the crème fraîche. Blitz the soup until smooth using a hand blender, or do this in batches in a jug blender. Taste for seasoning and adjust as needed. Once cooled, will keep frozen for up to three months. Defrost thoroughly before reheating until piping hot. Ladle the soup into bowls, then top with crispy leeks, if you like (see tip, below) and a grinding of black pepper to serve.