This study introduces the Balanced Bending Stiffness (BBS) method, a novel experimental approach to measure the intrinsic Mode-I interfacial fracture toughness (G_IC) in overmolded hybrid composites. Traditional testing methods for these asymmetric systems are complicated by inherent stiffness mismatches that couple opening and shearing failure modes, requiring complex post-analytical corrections. The BBS method addresses this challenge by engineering physically balanced Asymmetric Double Cantilever Beam (ADCB) specimens through comparative stiffness matching, isolating pure mode-I failure conditions and enabling direct toughness measurement. The method is validated using a glass fiber–reinforced polypropylene (GF/PP) system, with parametric studies investigating the effects of fiber content and processing temperatures on interfacial toughness for short-fiber (SFT) and long-fiber (LFT) thermoplastics. Results reveal that higher fiber content and substrate preheating significantly enhance toughness, with particularly strong results for LFTs due to fiber bridging. This work provides a framework for material characterization and insights into optimizing overmolded composite interfaces.