Melt the butter in a large saucepan. Add the onion and cook over moderate heat until softened, about 5 minutes. Add 2/3 cup of the flour and cook, stirring, for 1 minute. Transfer the paste to a large heatproof bowl and slowly whisk in the milk to form a smooth slurry. Return the mixture to the saucepan and cook over moderate heat, stirring, until bubbling and very thick, about 6 minutes. Add the tuna to the white sauce along with the carrots, celery root, salt and pepper. Scrape the croquette mixture into a lightly oiled medium bowl and refrigerate until chilled and firm, at least 4 hours, or preferably overnight.