  We report on the realization of a Fermi-Fermi mixture of ultracold atoms that
combines mass imbalance, tunability, and collisional stability. In an optically
trapped sample of $^{161}$Dy and $^{40}$K, we identify a broad Feshbach
resonance centered at a magnetic field of $217\,$G. Hydrodynamic expansion
profiles in the resonant interaction regime reveal a bimodal behavior resulting
from mass imbalance. Lifetime studies on resonance show a suppression of
inelastic few-body processes by orders of magnitude, which we interpret as a
consequence of the fermionic nature of our system. The resonant mixture opens
up intriguing perspectives for studies on novel states of strongly correlated
fermions with mass imbalance.
