  A mixed action composed of valence quark flavours regularized with a
fully-twisted tmQCD action and of $N_f=2+1$ flavours of non-perturbatively
${\rm O}(a)$-improved Wilson sea quarks is described. Two procedures for the
matching of sea and valence quark masses are discussed. We report about a
comparison of the continuum-limit scaling of pseudoscalar meson observables and
of quark masses using the sea and valence actions.
