Growing nonlinear loads in modern grids cause higher harmonics, prompting new grid codes that require inverter-based sources to participate in harmonic mitigation. This paper presents a novel approach to mitigate harmonics in microgrids by utilizing the available capacity of Photovoltaic (PV) systems, merging clean energy production with enhanced power quality. The proposed control strategy employs Extremum Seeking Control (ESC) due to its maturity and ease of implementation. The ESC minimizes the Point of Common Coupling (PCC) voltage Total Harmonic Distortion (THD) when load and grid impedances are unknown. Controller parameter tuning depends on available inverter capacity and desired control performance. To mitigate PCC harmonic voltage, the PV injects a suitable harmonic current, in phase with the load harmonic current, ensuring maximum cancellation. The injected harmonic current magnitude remains lower than the load current magnitude, considering the available inverter capacity. Simulations in PSCAD validate this technique, demonstrating significant PCC voltage THD reduction without prior knowledge of the load current harmonic. Specifically, the voltage THD and the current THD with the ESC, are reduced to 0.29% and 2.40%. Performance comparisons with an Active Power Filter (APF) exhibit the ESC has superior performance (APF reduced voltage THD to 0.69% and current THD to 5.61%). These results demonstrate the ESC's effectiveness and potential as a viable solution for harmonic mitigation in microgrids, offering a significant improvement in power quality and supporting the increasing integration of renewable energy sources.