We report on the design and the numerical and experimental characterization of an all-dielectric reflecting focusing metasurface (metamirror) which does not have a back reflector, but effectively reflects incident waves with the desired reflection phase gradient. The profile of the reflection phase can be tuned independently for both sides of the single-layer subwavelength-thick metamirror by properly selecting dimensions of its dielectric inclusions. Such a feature stems from the bianisotropic omega properties of the inclusions. To demonstrate independent control of the reflection phase, we have designed a metamirror focusing normally incident plane waves at different focal distances being illuminated from the opposite sides. The proposed two-sided metamirror can find applications in antennas, diffraction gratings, and complex holograms. The absence of conducting elements makes it a perfect candidate for optical applications requiring asymmetric wave-front control.