A 62-year-old postmenopausal woman presents with progressive abdominal distension and bloating for 3 months. On examination, she has ascites and a palpable abdominal mass. Transvaginal ultrasound shows a complex ovarian mass (8 cm) with solid and cystic components, and free fluid in the pelvis. CA-125 is elevated at 320 U/mL. What is the most appropriate next step in management?