A 58-year-old man who smokes and drinks heavily presents with a painless lump below the angle of his left jaw that he first noticed six weeks ago and which has slowly enlarged. He has no fever, weight loss or night sweats, but mentions that his voice has been "a bit rough" for a couple of months. On examination there is a firm, non-tender 3 cm mass deep to the upper sternocleidomastoid muscle. The oral cavity and oropharynx appear normal; the rest of the neck and the axillae have no palpable nodes.
Which one of the following is the most appropriate initial investigation?