Season and sweat the sliced shallots, red onion and white onions on medium heat for 15 minutes.
Add 1 tbsp unsalted butter and 1 tbsp brown sugar and continue caramelizing for 10 minutes.
Grate in 2 cloves garlic and add some thyme.
After a further 10 minutes, drizzle in 1 tbsp balsamic vinegar, 2 tbsp sherry vinegar and 200 ml red wine.
Reduce for 5-7 minutes until almost gone, then add 500 ml beef stock and continue reducing until thick and voluptuous.
Cover 3-4 maris piper potatoes in a little olive oil and sea salt, then bake at 180°C for 1 hour. Remove skins and pass through a ricer or sieve.
Add the mashed potatoes to a warm pan along with 3 tbsp unsalted butter, sea salt, white pepper and 50 ml whole milk, and mix together until smooth and creamy.
Cook the sausages over medium heat in a pan laced with a little neutral oil, turning occasionally until golden brown, about 15 minutes. Let rest for 5 minutes.
Plate the mashed potatoes and top with the sausages and the red wine and onion gravy. Garnish with chives.
