  Optical nanoresonators are fundamental building blocks in a number of
nanotechnology applications (e.g. in spectroscopy) due to their ability to
efficiently confine light at the nanoscale. Recently, nanoresonators based on
the excitation of phonon polaritons (PhPs) $-$ light coupled to lattice
vibrations $-$ in polar crystals (e.g. SiC, or h-BN) have attracted much
attention due to their strong field confinement, high-quality factors, and
potential to enhance the photonic density of states at mid-infrared (IR)
frequencies. Here, we go one step further by introducing PhPs nanoresonators
that not only exhibit these extraordinary properties but also incorporate a new
degree of freedom $-$ twist tuning, i.e. the possibility to be spectrally
controlled by a simple rotation. To that end, we both take advantage of the
low-loss in-plane hyperbolic propagation of PhPs in the van der Waals crystal
$\alpha$-MoO$_3$, and realize dielectric engineering of a pristine
$\alpha$-MoO$_3$ slab placed on top of metal ribbon grating, which preserves
the high-quality of the polaritonic resonances. By simple rotating the
$\alpha$-MoO$_3$ slab in the plane (from 0 to 45$^{\circ}$), we demonstrate via
far- and near-field measurements that the narrow polaritonic resonances (with
quality factors Q up to 200) can be tuned in a broad range (up to 32 cm$^{-1}$,
i.e up 6 ~ times its full width at half maximum, FWHM ~ 5 cm$^{-1}$). Our
results open the door to the development of tunable low-loss nanotechnologies
at IR frequencies with application in sensing, emission or photodetection.
