Stir the shallot, parsley and garlic together in a bowl and set aside.
Heat the oil in a frying pan and fry the bacon over a low heat for 2-3 minutes, or until golden-brown. Add the lettuce to the pan, cut-side down and the mushrooms and chicken stock and cook for 15-20 minutes.
Meanwhile, heat a frying pan over a low heat and add half of the butter. Season the flour to taste with salt and freshly ground black pepper. Dust the liver slices in the seasoned flour and add to the butter once it is foaming. Cook for 2-3 minutes, then turn over and cook for another minute. Remove the liver from the pan and set aside, then deglaze the pan with the balsamic vinegar, scraping up any browned bits. Add the remaining butter and beef stock and continue to cook until the sauce is well combined and thickened slightly. Season, to taste, with salt and freshly ground black pepper.
To serve, spoon the braised lettuce onto serving plates, top with the liver and spoon over the balsamic sauce. Spoon over the shallot mixture.