We introduce an optical microresonator consisting of a planar waveguide terminated by metallic mirrors. The resonator was fabricated on a silicon-on-insulator platform, and its optical performance was theoretically and experimentally investigated. The demonstrated device had dimensions of 200 mumx40 mum and exhibited a quality factor of about 1000 and a free-spectral range of about 8 nm. Application to high-throughput, label-free biochemical sensing is considered, and optimization with respect to the surface sensitivity is carried out. The optimized sensitivity makes it possible to detect subnanometer layers of molecules adsorbing to the surface of the resonator.