Heat the oil in a large deep frying pan over moderate heat. Add the onions and and cook for 10 to 15 minutes, stirring frequently, until the onions are light gold.
Add the capsicum, garlic and sweet potato to the pan, cover and cook for 5 minutes, or until the sweet potato begins to soften.
Stir in the stock, thyme and a pinch of salt. Bring to the boil then reduce the heat, cover and simmer for 10 minutes or until the sweet potato is just tender. Stir in the peas and corn kernels.
Place the fish fillets on top of the vegetables, cover and cook gently for 8–10 minutes, or until the fish is cooked through. Season to taste with black pepper, then serve immediately sprinkled with fresh thyme leaves.