Preheat the grill to high. Melt the butter in a large frying pan and fry the onions and sugar over a medium heat, stirring frequently, for 7-8 minutes until nicely browned.
Season with salt and pepper, add the balsamic vinegar and leave it to bubble up for a few seconds. Keep warm.
Grill the bacon until beginning to crisp at the edges. Keep warm.
Season the liver on both sides with salt and pepper and then coat lightly in the seasoned flour. Pat off the excess.
Heat another large frying pan over a high heat. Add the oil and then the slices of liver and cook them over a high heat for 30 seconds on each side, the object being to have the liver nicely browned on the outside, yet still pink and juicy on the inside.
To make the root vegetable and rocket purée, cook the swede, carrots and potatoes in boiling salted water for 20 minutes, until tender. Then drain, place in a blender with the butter and blitz into a smooth purée.
Finally, return the purée to the pan and leave on a gentle heat, until the mixture has dried out. Add the rocket, salt and pepper and serve with the pan-fried-liver, bacon and caramelised balsamic onions.