Slow cooker vegetarian tacos

Name: Slow cooker vegetarian tacos
Ingredients: 2 onions (1 red and 1 any colour), kept seperate and finely chopped; 300g/10½oz frozen soya mince; 1 tbsp ground cumin; 1 tsp paprika (preferably smoked, hot if you like it really spicy); ¼ tsp dried chilli flakes; 400g tin red kidney beans, drained and rinsed; 400g tin whole plum tomatoes; 2 tomatoes, finely chopped; 8 soft white or wholemeal flour tortillas; 300g/10½oz plain or Greek-style yoghurt; 1 bag chopped iceberg lettuce (or ½ iceberg lettuce, shredded); salt and freshly ground black pepper