Electrochemical energy storage devices such as supercapacitors are a promising alternative technology that can deliver high-power and enable high load currents. Supercapacitors are better than batteries for high and low temperature charge-discharge performance. Currently, several biomass resources are being proposed as raw material to obtain carbon electrodes for supercapacitors, mainly due to their abundant availability and low cost. In this context, the production of onion waste has increased world-wide in recent years. Onion waste includes skin, outermost layers, roots and stems. In this work, onion waste-derived biocarbons (OWB) have been obtained from the pyrolysis of skin and outermost layer at 400, 600, and 800 °C (labeled as OWB400, OWB600 and OWB800), followed by chemical activation with ZnCl₂, and a novel microwave-assisted functionalization treatment with methanol. The chemical composition of the OWBs is mainly carbon (< 86 wt. %), while their structure is amorphous in nature as expected. Furthermore, high surface specific areas (SSA) up to 1611 m²g^-1 have been determined, with mesoporous around 20 nm. The electrochemical characterization indicates that the specific capacitance increases at higher pyrolysis temperature. Therefore, OWB800 shows a higher electrochemical performance (capacitance of 309.32 F g^-1) compared to OWB400 and OWB600, which can be ascribed to the pyrolysis temperature and the functionalization with methanol. Thus, the experimental procedure proposed here produces OWB with mesoporous structure and surface functional groups, characteristics that promote a high capacitance performance.