Preheat the oven to 180° C , 170° C fan assisted, Gas mark 4.
Heat the oil in a frying pan and fry the sausages until browned, add the onions and cook for a further 3-4 minutes until softened. Place in an ovenproof casserole dish with the mushrooms and pepper.
Blend the contents of the sachet with the water and add to the dish. Cover and cook for 1 hour.
Enjoy with creamy mashed potato and seasonal vegetables.