A 36-year-old man presents to his family physician with loud snoring, witnessed apnoeas, non-restorative sleep, and daytime sleepiness. His BMI is 34 kg/m² and neck circumference is 44 cm. Using a STOP-BANG questionnaire, he scores high risk for obstructive sleep apnoea (OSA). According to Canadian sleep and primary care guidance, what is the most appropriate next diagnostic step?