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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.
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