A thermally insulated vessel for cooking, serving, and storing of foods and beverages, particularly adapted for use in microwave cooking units. A double-walled vessel is provided, including an outer container and a removable inner container, both having substantially similar shapes. The inner container is supported by a grooved flange extending outward from its upper rim which rests on the upper rim of the outer container. The inner container and the outer container are separated by a narrow gap, forming an enclosed air cavity. The flange groove and rim are designed to allow air to escape from the cavity to relieve the pressure caused by hot foods or beverages in the vessel, and to resist air and liquid flow into the cavity as the vessel cools. A raised lid is further provided, fitting over the inner container, and having a flat top and a recessed handle, to allow several vessels to be stacked on each other for cooking or storing.