Cooling apparatuses and methods are provided for immersion-cooling one or more electronic components. The cooling apparatus includes a housing at least partially surrounding and forming a fluid-tight compartment about the electronic component(s), and a boiling fluid mixture of first and second dielectric fluids within the fluid-tight compartment, with the electronic component(s) immersed within the mixture. A condensing fluid is also provided within the fluid-tight compartment, and is immiscible with the boiling fluid mixture. The condensing fluid has a lower specific gravity and a higher thermal conductivity than the boiling fluid mixture, and facilitates condensing of vaporized boiling fluid mixture. A cooling structure is provided within the compartment, and includes a condensing region and a sub-cooling region, with the condensing region being in contact with the condensing fluid, and the sub-cooling region being in contact with the boiling fluid mixture. The cooling structure facilitates heat removal from the fluid-tight compartment.