Peel the sweet potatoes and chop them into wedges. Toss with olive oil, chilli powder, paprika if using, oregano and salt. Roast for 45 mins or until golden and slightly crispy.
If using an ear of corn, boil in salted water in a pot or pressure cooker. Run a knife down the length of the cob and as close to the center as possible to release the kernels. If using frozen corn, boil in salted water until tender, about 5-7 mins.
Add half the amount of wedges to an ovenproof dish. Top with half the amount of corn, red peppers, tomatoes and onions. Add half the amount of cheese. Repeat one more time in the same order starting with wedges and ending with cheese.
Bake in a 180 C oven for about 10 mins or until the cheese has completely melted through. Top with coriander leaves.
Avocado crema: blend the avocado, coriander leaves, lime juice, green chilli and salt to a smooth sauce. Taste and adjust lime juice.
Dollop the crema over the top and serve immediately. This dish can be assembled up to a day in advance. Bake just before serving.