Heat the olive oil in a large, heavy-based saucepan over a medium heat. Add the garlic cloves and plum tomatoes. Sweat for 5-6 minutes, stirring occasionally, until soft, then add the vegetable stock. Bring it to a simmer, then arrange the tuna steaks in the saucepan, and cover.
Cook gently for 8-10 minutes until cooked through. Remove from the saucepan and keep warm to one side covered with aluminium foil. Add the raisins, pine nuts and sliced parsley to the saucepan, stirring well to combine.
Adjust the seasoning as necessary then stir in the pitted black olives and baby capers. Keep the sauce warm to one side as you bring a large saucepan of salted water to the boil. Blanch the courgette for 2-3 minutes until just soft, then drain. Arrange a tuna steak on each serving plate, then spoon the sauce around it. Garnish with the courgette and a sprig of parsley leaves before serving.