1. Heat the oil in a deep non-stick frying pan. Add the sausages and fry them over a moderate heat, turning occasionally, for 10 minutes, or until they are lightly browned all over.
2. Meanwhile, start cooking the potatoes for the mash. Place the potatoes in a saucepan and pour over boiling water to cover by 5 cm. Bring back to the boil, then reduce the heat and cook for 15–20 minutes, or until the potatoes are very tender.
3. Add the onions to the sausages and cook for a further 5 minutes, or until they are golden and the sausages are nicely browned all over. Remove the sausages to a plate and set aside. Drain off the excess oil from the pan.
4. Add the garlic, mushrooms and capsicum to the pan and cook gently for a few minutes until softened. Pour in the stock and wine and add the redcurrant jelly and 2 sprigs of thyme. Season with salt and pepper to taste. Mix the cornflour with 1 tablespoon cold water and stir into the liquid in the pan. Bring to the boil, stirring until lightly thickened, then reduce the heat and return the sausages to the pan. Simmer gently for 10 minutes.
5. Drain the potatoes, shaking the colander or sieve to remove any excess water, and return them to their pan. Heat the milk in a small saucepan until hot, pour over the potatoes and mash until smooth. Beat in the oil, parsley and salt and pepper to taste.
6. To serve, divide the mash among warmed plates and top with the sausages, vegetables and sauce (discarding the thyme sprigs). Sprinkle with the leaves from the remaining thyme sprig and serve immediately.