This paper presents a novel asymmetric U-shaped refractive index sensor, which is based on a MIM waveguide and coupled with a U-shaped resonator, which integrates a ring, a circular cavity, and two rectangular cavities (URRCTR), in addition to an inverted rectangular nanostructure. The efficiency of the proposed sensor was investigated and optimized through the FEM. Simulation results indicate that the interaction between the broadband mode supported by the inverted square-shaped structure on the primary waveguide and the confined narrowband mode of the URRCTR resonator generates a distinct asymmetric feature in the transmission profile, a characteristic indicative of Fano resonance. The geometric parameters of the structure are crucial for tuning the Fano resonance features. Through systematic optimization, the sensor achieves a sensitivity of 3480 nm/RIU and a figure of merit (FOM) of 55.23. Due to its high sensitivity, compact footprint, and favorable temperature-dependent properties, the presented sensor reveals considerable promise for various applications in integrated photonic sensing.