Preheat your slow cooker to the highest setting and place and heat a little oil in a large frying pan over a medium-high heat.
Brown your sausages evenly in the hot pan, then add in your red onion and cook for 5 minutes, stirring, until the onions are soft and caramelised.
Transfer the sausages and onion into your pre-heated slow cooker and combine the sachet of Colman’s Full of Flavour Sausage Casserole Seasoning Mix with water, then pour over.
Add in your cannellini beans, then stir, top with rosemary and cover, leaving for 2 hours to cook.
Half an hour before your casserole is ready, start making your Yorkshire puddings. Place an 8 inch cake tin in the oven with a little vegetable oil, then preheat the oven to 200c. Whisk the eggs into the plain flour, then slowly add in the milk, whisking to a smooth batter. Pour this into the hot cake tin, about ¼ full and bake for 20 minutes, until fully risen.
Spoon the sausage casserole into the Yorkshire Pudding bowls and serve with a dollop of Colman’s English Mustard (optional).