Avocado and sweet potato tacos

Name: Avocado and sweet potato tacos
Prep time: 25 min
Total time: 25 min
Cuisine: American, Mexican, Latin American
Ingredients: 1 sweet potato, peeled and cubed; 2 tbsp olive oil; ó teaspoon chilli powder; 1 x 380g carton black beans, drained and rinsed; 125g Greek yogurt; 1 small avocado, peeled, stone removed, and roughly chopped; 1 garlic clove, chopped; juice of 1 lime, plus lime wedges, to serve; 12 small tortillas (or 4 large); coriander leaves, to serve; 1 small avocado, diced; 2 spring onions, trimmed and sliced; 50g feta cheese, crumbled