β-FeSi<sub>2</sub> is a semiconducting transition metal silicides that has appealing thermoelectric properties, including a high Seebeck coefficient and low thermal conductivity, with constituents that are inexpensive, non-toxic, and oxidation resistant. However, poor electrical conductivity reduces efficiency, and requires additional materials engineering to become a viable thermoelectric material. Performance might be improved by creating a fine-scale nanocomposite of β-FeSi<sub>2</sub> and Si<sub>1-x</sub>Ge<sub>x</sub> phases. SiGe may improve overall carrier transport, while decreasing thermal conductivity due both to bulk alloy scattering and enhanced boundary scattering at β-FeSi<sub>2</sub>/Si<sub>1-x</sub>Ge<sub>x</sub> heterointerfaces. Microstructure and composition can be hierarchically controlled by exploiting the eutectic solidification (L → α-FeSi<sub>2+δ</sub> + Si<sub>1-y</sub>Ge<sub>y</sub>) and subsequent eutectoid decomposition (α-FeSi<sub>2+δ</sub> → β-FeSi<sub>2</sub> + Si<sub>1-z</sub>Ge<sub>z</sub>). We analyzed the measured thermal conductivities via a series thermal resistance model, and show that thermal boundary conductance is reduced by at least an order of magnitude when the Si<sub>1-x</sub>Ge<sub>x</sub> microconstituent is enriched from 0 to 30 at% Ge.