Preheat your oven to 200°C (400°F) and line a baking tray with parchment paper.
Place the sausages on the tray, drizzle with olive oil, and bake for 10 minutes on each side until golden brown. Alternatively, cook them in a pan over medium-high heat for 4 minutes on each side. For crispy skin, bake in the oven.
Peel the whole potatoes and place them in a large saucepan or pot. Fill it with enough cold water to cover the potatoes by an inch. Bring to a boil over high heat and cook for 22-24 minutes until soft but not falling apart. Drain the potatoes through a colander.
Mash the potatoes using a ricer or masher until smooth. Add warmed milk, butter, salt, and ground white pepper, then mix well until creamy.
Onion Gravy
Melt the butter in a saucepan over medium heat. Add the sliced onions and sugar, and cook for 30 minutes, stirring regularly, until caramelised.
Increase the heat to medium-high, add the red wine and rosemary, and bring to a boil. Reduce the heat to medium and simmer for 10 minutes. Add beef stock, Worcestershire sauce, Dijon mustard, salt, and pepper, then bring to a boil. Reduce the heat to low, cover, and simmer for 20 minutes. Remove the rosemary.
Add balsamic vinegar and whisk the cornflour (starch) with cold water. Then gradually add it to the pan, whisking constantly until thickened. Simmer for another minute or two. Stir in the cold butter until melted. Season with salt and pepper to taste and add more mustard if you like.
Serving
Spoon a generous bed of mash onto each plate. Place sausages on top and pour over the onion gravy. Garnish with flat-leaf parsley.
Chef's Tips
Start the gravy first — the onions need 30 minutes of unrushed cooking. Everything else fits around that timeframe.Use a floury potato — Kestrel, Russet or Yukon Gold. Waxy potatoes like Nicola don't break down properly and give you a gluey, starchy mash.Warm the milk before adding it to the mash — cold milk cools the potatoes down and makes the butter harder to incorporate, leaving you with a less creamy result.Baking the sausages gives a more even, golden result than pan-frying. For crispy skin with no splitting, oven is the way to go.The cold butter stirred in at the end of the gravy is a classic finishing technique — it adds richness and gives the gravy a glossy, restaurant-quality sheen.
Storage & Reheating
Store mash and sausages in the fridge for up to 3 days in airtight containers. Store gravy separately.The onion gravy freezes well for up to 3 months. Cool completely, portion into containers or freezer bags and freeze flat. Thaw overnight in the fridge.Reheat mash in a saucepan over low heat with a splash of milk, stirring until smooth and hot.Reheat sausages in the oven at 180°C (355°F) for 10 minutes or in an air fryer for 5–6 minutes. Reheat gravy in a saucepan over low heat, stirring, adding a splash of stock or water if it has thickened too much.