To caramelise the onions: heat up a splash of olive oil in a pan and begin cooking the sliced onions on a low-medium heat with a pinch of salt for around 15 to 20 minutes, or until golden. Add the brown sugar and 1 tablespoon of the balsamic vinegar and cook for a further 10 minutes on a low heat, or until sticky and caramelised. Place in a bowl, covered, and put in the fridge until needed.
Heat up a little olive oil in a frying pan over medium heat. Add sausages and fry for 10 to 15 minutes, or until golden brown. After this, tip in the red grapes, garlic and caramelised onions and cook for a further 5 minutes until the grapes begin to soften and the onions reheat.
Pour in the final 2 tablespoons of balsamic vinegar and swirl around the pan until all the contents become really sticky and all the sausages are completely cooked throughout.
Serve with creamy mash and season with heavy helpings of salt and pepper!