Red lentil dahl with sweet potato

Ingredients (serves 4) 1 cup (200g) red lentils, rinsed 1 sweet potato, chopped into 2cm pieces 1 red onion, halved, finely chopped 3 garlic cloves, finely chopped ½ long red chilli, seeds removed, finely sliced 2 tablespoons mild curry powder 2 tablespoons tomato paste ½ cup (150g) low-fat thick Greek-style yoghurt 2 spring onions, finely…