  We realise a turbulent cascade of wave excitations in a homogeneous 2D Bose
gas, and probe on all relevant time and length scales how it builds up from
small to large momenta, until the system reaches a steady state with matching
energy injection and dissipation. This all-scales view directly reveals the two
theoretically expected cornerstones of turbulence formation -- the emergence of
statistical momentum-space isotropy under anisotropic forcing, and the
spatiotemporal scaling of the momentum distribution at times before any energy
is dissipated.
