Preheat the oven to 180˚C, gas mark 4. Heat ½ tbsp oil in a large ovenproof frying pan or casserole dish over a medium heat and add the onions, thyme and sausages. Fry for 10 minutes, stirring regularly, until the sausages are golden.
Meanwhile, put the root vegetables in a steamer and steam for 20 minutes (or boil in water for 20 minutes, then drain and leave to steam dry for a couple of minutes).
Sprinkle the flour into the pan with the sausages and fry, stirring, for 1 minute more. Pour in the Guinness and simmer for a couple of minutes to burn off the alcohol. Add the chicken stock, honey and mustard and stir until combined.
Bring to a simmer, then put in the oven for 15 minutes until the sauce has reduced and the sausages are cooked through.
Check the root vegetables are tender by piercing with a cutlery knife; it should slip in easily. Transfer to a bowl. Add the remaining 1 tbsp oil, season and mash with a potato masher. Serve with the sausages, onions and gravy.