In this paper, we show that the optical metamaterial dispersion can be controllably tailored to produce an ultrathin, broadband filter with a flat transmission band and near-constant in-band group delay in the 3 ∼ 3.5 μm midwave infrared window. It is achieved by strategically incorporating deep-subwavelength nano-notches into a fishnet structure. The deep-subwavelength inclusions, which have critical dimensions of less than 1/15 ± 1/100 of the shortest design wavelength, can perturb the mode patterns and strength of the unmodified fishnet nanostructure resonances, thereby shaping the dispersion in the effective medium properties to meet the specific design metrics. The designed free-standing optical metamaterial filter was fabricated and characterized, showing strong agreement between measured and simulated results. This powerful design approach will dramatically expand the opportunities to create new and practical broadband metamaterial-enabled optical devices and components.