Preheat the oven to 200°C and line a large baking tray with paper.
Peel and dice the sweet potatoes, place them in a single layer on the greased baking tray, drizzled with 2 tablespoons of olive oil and sprinkled with the chilli con carne spice. Bake for 30-40 minutes, tossing occasionally. The sweet potato is cooked when it is soft inside and crisp outside.
Meanwhile, heat the olive oil in a large pan, add the garlic, black beans and taco spice. Stir a little until the garlic and spices become fragrant, then add the tomatoes. Increase heat first, then simmer, uncovered for 20 minutes and stirring occasionally, until the sauce has reduced and thickened.
Pick the leaves from the coriander, slice the avocado and shred the lettuce.
To assemble the tacos, fill each tortilla with some lettuce, black beans and avocado, finishing with crispy sweet potatoes.
Garnish with coriander and serve with lime wedges and sour cream.