This letter presents a novel use of a waveguide circuit consisting of a two-ring resonator-assisted asymmetrical Mach-Zehnder interferometer, which realizes a narrow-passband filter in the context of microwave photonics (MWP). The filter principle is an alternative to the coupled resonator induced transparency and features easy implementation and robust performance. In the experimental demonstration, such a circuit fabricated in SiO2/Si3N4 waveguide technology exhibits a narrow transmission window surrounded by a region of significant suppression. The transmission window features a -3-dB bandwidth of 1 GHz and a -20-dB bandwidth of 3.5 GHz, equivalently a 20-dB roll-off enhancement of three times as compared with a regular add-drop ring resonator. In addition, the investigated waveguide circuit features full reconfigurability based on tunable phase shifters and power couplers. This allows the proposed filter functionality to be combined with other functionalities in a common device, which is of high interest for the realization of flexible on-chip MWP signal processors.