An optical assembly with mounting provided to effectively transfer heat away from an optic, such as a slab or waveguide amplifier or laser disk, while limiting internal stresses. The assembly includes an optic with a planar surface. A heat sink is positioned in the assembly with an upper surface next to the planar surface of the optic. The upper surface of the heat sink comprises a recessed surface defining a reservoir for containing a compliant heat transfer material. The assembly may further include a volume of the heat transfer material, such as a liquid metal or thermally conductive gel, in the reservoir of the heat sink. In one embodiment, the optic is a slab amplifier with a reflective coating or layer that directly contacts the heat transfer material in the heat sink reservoir or a foil or membrane is provided between the heat transfer material and the slab.