Blanch the cabbage in a large pan of salted water for 3 minutes then drain.
Heat 1 tablespoon of the oil and the butter in a frying pan and cook the onion until softened. Add the carrot and celeriac and cook for 2 minutes. Pour in the cream and season with salt and pepper. Add the cabbage to the pan and cook until the volume of the liquid has reduced by a third.
Meanwhile, heat 1 tablespoon of the oil in a frying pan and fry the liver slices for 1½ minutes on each side. Season with salt and pepper.
Oil a griddle pan with the remaining oil and place over a high heat. Add the leeks and griddle until slightly charred. Season with salt and pepper.
To serve, divide the cabbage between two serving plates. Add the liver and top with the leeks. Pour over the gravy and garnish with the watercress.