Preheat oven to 180ºC. Bring a pot of salted water to the boil and add the potatoes.
Heat a glug of olive oil in a large oven-proof frying pan on medium-high heat. Add the sausages, gently pricking each with a toothpick or fork to prevent the casings from bursting. Smash the garlic cloves and add to the pan along with the rosemary. Cook the sausages for a few minutes until browned, then add the butter. Once the butter has melted, baste the sausages to coat and the place the pan in the oven to cook.
Place a seperate non-stick frying pan on medium-high heat and add a glug of olive oil. Add the red onion and cook for about 1 minute before adding the sugar and a pinch of salt. Add the butter and stir to coat. Then add the Balsamic vinegar and Worcestershire sauce. Cook for a couple of minutes until reduced slightly, then add the beef stock. Bring to a boil and reduce to a gravy sauce consistency.
Meanwhile, once the potatoes have softened, drain using a colander and return the pot to medium heat. Add the strained potatoes back into the pot and mash using a masher. Reduce the heat to low, add the olive oil and stir using a silicone spatula. Add the chives, mustard and a pinch of salt. Stir until evenly combined.
Evenly divide the mashed potato onto each serving plate. Place the sausages atop the potato and top with the onion gravy. Serve immediately.