A 50-year-old man presents with fever, weight loss and night sweats for 3 months. Examination reveals splenomegaly and generalised lymphadenopathy. Hb is 90 g/L, WBC 25 × 10⁹/L with lymphocytosis, and platelets 80 × 10⁹/L. What is the most appropriate initial diagnostic investigation to establish the nature of his lymphoproliferative disorder?