Preheat oven 220°C fan forced. Cut the sweet potatoes in half crossway then into wedges lengthways. Place onto a large, greased baking tray. Drizzle with oil and sprinkle over the fajita. Turn to coat. Roast 20 minutes, turning once until golden and tender.
Combine all the dressing ingredients in a large bowl. Season then whisk until well combined. Remove 1 tablespoon to a jug.
While sweet potato is roasting, cook corn on a greased barbecue grill plate, turning occasionally, for about 10 minutes, or until lightly charred. Remove to a board, cut kernels from cobs. Add corn, black beans, tomato, avocado and onion to the dressing. Stir to combine.
Spoon corn mixture over the roasted sweet potato. Pour over the reserved dressing. Toss gently to combine. Scatter with coriander. Serve.