Put the onion, lemon zest, juice and 1 tbsp
oil into a small bowl, add salt and pepper to
taste, then leave to soak for 5 mins until the
onion has softened a little. Pour the hot stock
over the couscous in a large bowl, cover with
cling film and leave for 10 mins until the
stock has been absorbed.
Season the tuna steaks, brush with oil, then
pat the mustard powder over both sides. Once
the couscous is ready, add the parsley and
capers to the onions, stir well, then mix into
the couscous with a fork. Heat a griddle pan
and sear the tuna for 1 min on each side
until medium-rare. Serve the tuna with the
couscous and lemon wedges on the side.