Preheat the oven to 220 degrees C. Heat 1 teaspoon oil in a small saucepan. Add the onion and cook over a medium heat for 4 minutes or until softened. Add the tomatoes and their juice, oregano, sugar and pepper. Bring to the boil, then leave to bubble for 10 minutes, stirring occasionally.
Place the pizza bases on 2 baking trays. Spread 1 tablespoon tomato puree over each base. Spoon the tomato mixture over the pizzas, then lay the tuna over the top. Add the capers and sliced olives, and drizzle the remaining oil over the top.
Bake the pizzas for 10 minutes or until the bases are golden-brown and crisp. Garnish with the torn basil leaves and serve.