In a pot, boil water. Add the potatoes to it and cook them.
When the potatoes are cooked, drain the water. In a pot placed over heat, add the potatoes and mash them. Then, add 2 tablespoons of olive oil to it. Also, add chives, mustard, and smoked salt and mix them well.
Now, heat oil in a pan and add sausages to it. Then, add smashed garlic and rosemary sprigs to the pan. Add about 4 knobs of butter to the pan and cook the sausages in it. Add another knob of butter to the pan and place the pan in the oven and cook them for some time.
Take another pan and heat oil in it. Add onions to it and cook them for half a minute. After that, add some sugar and salt to them and add 4 knobs of butter to the pan and let it melt. Add balsamic vinegar and Worcestershire sauce to it and cook it until reduced
When the mixture is reduced, add beef stock to it. Add some butter and let it melt.
Now, place the mashed potatoes on a serving plate. Place the sausages over the potatoes and top them with the onion mixture. Serve your bangers and mash.