Preheat the oven to 190°C, fan 170°C, gas 5. Place the sausages in a roasting tin and cook in the oven for 25-30 minutes, turning twice, until browned and cooked through.
Meanwhile, place the potatoes and celeriac in a pan and cover with cold water. Bring to the boil, then simmer for 20 minutes. Drain well, return to the pan with the milk and 20g butter and mash until smooth. Season to taste.
To make the onion gravy, melt the remaining butter in a frying pan and fry the onion gently for 10-12 minutes.
Remove the sausages from the oven and drain on kitchen paper. Drain off all but 1 tbsp of the fat from the tin. Add that to the onion gravy, along with any sticky bits from the tray. Sprinkle the flour over the fried onion and cook for 1 minute then gradually stir in the beef stock. Simmer for 2-3 minutes until thickened. Serve the sausages with the mash and gravy.
For an adult kick, add 1 tbsp of creamed horseradish to the mash.