Preheat the oven to 200°C (400°F), Gas Mark 6. Heat the oil in a large frying pan and cook the sausages for 7–8 minutes, turning occasionally, until cooked through.
Meanwhile, cook the potatoes in a saucepan of lightly salted boiling water for 5–6 minutes until tender. Reheat the Yorkshire puddings in the preheated oven for 3–4 minutes, or according to packet instructions.
Place the dried onions and gravy granules in a jug, add the boiling water and stir until dissolved. Add the potatoes and onion gravy to the sausages and stir to heat through.
Arrange the Yorkshire puddings on 4 serving plates and divide the sausage mixture between them. Serve immediately with peas.