The quality factor Q significantly exceeds 104 below 55 K and slightly decreases for increasing fields, remaining 90% of Q(B=0) for B=7 T and T=2 K. These features allow to coherently couple resonant photons with spin ensembles at finite temperature and magnetic field. To demonstrate this, collective strong coupling regime was achieved by using the spin ensemble of a DPPH organic radical placed at the magnetic antinode of the fundamental mode: the in-plane magnetic field is used to tune the spin frequency gap across the single-mode cavity resonance at 7.78 GHz, where clear anticrossings are observed with a splitting as large as ∼82 MHz at T=2 K. The spin-cavity collective coupling rate is shown to scale as the square root of the number of active spins in the ensemble.