Put your potatoes, broccoli and onion into the slow cooker and pour in the vegetable stock. In a 5 quart pot it should come to just below the maximum fill line but keep an eye on this. Cook on low for 6-8 hours, or high for 3-4. An hour before serving, roughly crumble or chop your blue cheese and add it to the slow cooker. Stir for a few moments to start the process of it melting in, before replacing the lid. After half an hour, lift and stir through to incorporate the melted cheese into the soup and then use an immersion blender to completely puree it. Let cook for another half hour before serving.