Place the potatoes on a baking tray and roast for 25-30 minutes.
Meanwhile, heat the oil in a large frying pan and fry the sausages and onions for 15 minutes, covering the pan for the last 5 minutes.
Cook the broccoli in boiling water for 5-6 minutes until tender, then drain.
Whisk 300ml boiling water into the gravy granules and stir in the mustard. Heat the Yorkshires in the oven for 5-6 minutes, then fill with potatoes, sausages and onions, and serve with the gravy and broccoli.