Tip the onions and leeks into a slow cooker pot and add the potatoes, mixed herbs and stock. Season with salt and lots of pepper and stir well. Cover with a lid and cook on high for 4–5 hours or ...
To make mashed potatoes in advance and keep them warm, use your slow cooker (a Crock-Pot, for example). Use the low setting and keep potatoes in the slow cooker for no longer than four hours.