Mexican Chicken with Taco Shells

Preheat the oven to Gas Mark 4/180 C/350 F. Heat the oil in a saucepan and gently cook the onion and garlic for 5 minutes, until softened. Stir in the flour and cook for 2 minutes. Gradually stir in the tomatoes, chilli sauce and oregano. Bring to the boil and then simmer for 15 minutes, stirring frequently. Add the chicken to to the tomato mixture and heat through for a few minutes. Remove the pan from the heat and stir in the natural yogurt. Season to taste. Heat the taco shells in the oven for 2-3 minutes or until warmed through. Divide the shredded lettuce between the taco shells and fill with the chicken mixture. Sprinkle over the cheese and serve immediately.