To make the filling, cook the rice in a large saucepan of boiling water, following packet directions, or until tender. Drain and set aside.
Meanwhile, heat the olive oil in a large frying pan over medium heat. Add the onion and capsicum and cook, stirring occasionally, for 3-4 minutes or until softened slightly.
Add the tomatoes and the tomato paste and mix well. Stir in the red kidney beans, cumin, chilli powder, garlic powder and water. Bring to the boil over high heat. Reduce heat to medium and simmer, uncovered, stirring occasionally, for 10-15 minutes or until the sauce has thickened.
Increase heat to high, add the drained rice to the bean mixture and stir to combine. Cook, stirring constantly, for 2-3 minutes or until heated through.
Preheat oven to 180°C. Place the taco shells on 2 baking trays. Heat in preheated oven for 4-5 minutes or until heated through.
Spoon the bean and rice mixture into a serving dish. Serve with the taco shells, tomatoes, lettuce, cheese, and sour cream and guacamole, if using. Allow the family or your guests to fill the tacos themselves at the table.
