Make a marinade for the steak with cumin, coriander, olive oil and soy sauce.
Cook the quinoa in a pot of salted boiling water until just tender, 10 to 12 minutes. Drain and return to the pot to cool.
As the quinoa cooks, make the steak. Cook until an instant-read thermometer inserted in the thickest part of the steak registers 130° F, 4 minutes per side for medium-rare. Let rest for 5 minutes, then slice against the grain.
While the steak rests, add the mixed salad, feta, cooked quinoa, onions and cherry tomatoes.
Mix yoghurt with fresh herbs and lime juice.
Serve the steak with the quinoa salad and the yoghurt dressing.