Begin by browning the sausages in a pan, or you can utilize the sear function on your slow cooker if available. This step enhances the flavor of the sausages.
Once browned, transfer the sausages to the slow cooker, along with the sliced leeks, garlic purée, chicken stock, Worcestershire sauce, Dijon and wholegrain mustards, dried herbs, and seasonings. Stir well to combine all ingredients.
Set your slow cooker to HIGH and cook for 3-4 hours, or on LOW for 7-8 hours. This allows the flavors to meld beautifully.
About 10 minutes before serving, add the crème fraîche to the slow cooker and stir it through to create a creamy texture. Let it cook for an additional 10 minutes.
Serve your creamy sausage and leek casserole alongside fluffy mashed potatoes and tenderstem broccoli for a complete meal.