Heat the oil in a large pan and fry the onion about 5 minutes or until soft.
Add the crushed garlic and cook for a couple of minutes more.
Add the wine and cook for a few minutes then add the tomatoes and chickpeas.
Allow to cook gently for about 10 minutes.
Meanwhile, pour the boiling water over the couscous and cover for about 5 minutes.
Add the prepared couscous, herbs and capers to the tomato mix, season well with salt and pepper and set aside while you cook the tuna.
Season the tuna steaks with salt and pepper.
Heat the oil in a large pan and cook the tuna about 4 or 5 minutes each side: this will give you a rare steak, if you'd like it cooked more add a couple more minutes each side. Remember though that it will keep cooking after you take it off the heat.
Gently reheat the tomato mix for a couple of minutes, then divide it between 4 plates and serve a tuna steak on each.
Serve with a dollop of yogurt over each steak.