A 68-year-old man with a 40 pack-year smoking history presents with a 3-month history of persistent cough, occasional haemoptysis, and a 5 kg weight loss. A chest X-ray shows a left upper lobe opacity. A subsequent CT scan of the chest, abdomen, and pelvis confirms a 4 cm mass in the left upper lobe and identifies several enlarged mediastinal lymph nodes (largest 1.5 cm). There is no clear evidence of distant metastatic disease on the CT scan. He has no neurological symptoms or bone pain. Pulmonary function tests are pending. What is the most appropriate next investigation for staging this patient's non-small cell lung cancer?