Start this recipe by whizzing together the tuna ingredients (except the tuna) in a food processor to make a pesto. Take 1 tbsp of the mixture and rub onto the fresh tuna and allow to marinate for at least an hour. Meanwhile, mix together all the lemon yogurt ingredients and place in the fridge to chill for 30 minutes. In the meantime, you can also prepare the couscous by mixing together the couscous, harissa paste and the oil in a bowl and pour over the hot vegetable stock. Mix well, cover and allow to sit for 5 minutes. When you are ready to cook the tuna, heat up a griddle pan with a little oil and sear the tuna on both sides to your liking. To serve, place the couscous onto the center of 2 plates and lay over the grilled sliced vegetables. Spoon over a tbsp of the lemon yogurt and place the grilled tuna on top. Drizzle over the remaining coriander pesto on the tuna and also use to decorate the plate.