Prepare the vegetables.
Steam the kumara for 20 minutes or until tender, then mash.
While the kumara is steaming, preheat the grill (broiler) to hot, wipe the perch fillets with damp paper towel and pat dry.
In a small bowl mix the za'tar with enough of the olive oil to make a paste. Using a flat knife, smear the za'tar paste over each side of the fish.
Grill for 3 minutes each side.
Meanwhile, steam the spinach for 3 minutes or until wilted.
Serve the perch on the mashed kumara with steamed English spinach on the side.