Preheat the oven to 190°C, fan 170°C, gas 5. Put the sausages on a baking tray and cook in the oven for 30-40 minutes, turning occasionally, until golden and cooked through.
Meanwhile, cut the potatoes into quarters and put in a pan of cold water. Bring to the boil and cook for 15 minutes, until tender. Drain and mash with the butter and milk. Season with freshly ground black pepper, cover and set aside, keeping them warm.
Meanwhile, make the gravy. Heat the oil in a medium saucepan, add the onions and thyme, and cook over a gentle heat for 30 minutes, until soft and starting to caramelise.
Pour in the wine and let it boil down for 1 minute. Stir in the flour and redcurrant jelly, then gradually add the stock and bring to the boil. Cook for 5 minutes, until slightly reduced, then season with freshly ground black pepper.
Meanwhile, cook the peas in a pan of boiling water for 2 minutes, then drain and serve with the sausages, mash and gravy. Garnish with the remaining thyme leaves.