Using a large heatproof casserole dish add oil and fry sausages for 4-5 minutes. Add the red onions and fry for a further 2-3 minutes.
Mix together the red wine or stock, redcurrant jelly and fresh thyme and pour over sausages. Cover and place in oven to cook for 25-30 minutes. Remove sausages and stir in gravy granules to thicken. Serve with mashed potato.