A 45-year-old woman presents with exertional chest discomfort and dyspnoea when climbing stairs. She has hypertension and dyslipidaemia, and her pre-test probability of obstructive coronary artery disease is estimated to be intermediate. Her resting ECG and troponin are normal, and she is currently pain-free. Which investigation is the most appropriate next step to evaluate for stable coronary artery disease in this outpatient?