We develop, analyze, and demonstrate an optically-pumped semiconductor disk laser using an active mirror architecture formed by sandwiching the semiconductor gain membrane between two heatspreaders, one of which is coated with a high-reflectivity multilayer. Thermal modeling indicates that this structure outperforms traditional VECSELs. Employing an InGaAs/GaAs MQW gain structure, we demonstrate output powers of approximately 30 W at a center wavelength of λ ≈ 1178 nm in a TEM00 mode using an in-well pumped geometry.