Preheat the oven to 200°C, fan 180°C, gas 6. Heat 1 tablespoon oil and the butter in a large pan and fry the onion, mushrooms, peppers and leeks for 10 minutes, until slightly softened and beginning to turn golden.
Stir in the chicken fillets and cook for a further 5 minutes. Stir in the plain flour and dried herbs and cook for 1 minute before adding in the chopped tomatoes and stock. Bring to a boil and then simmer for 5 minutes, until starting to thicken. Season with freshly ground black pepper to taste.
Meanwhile, place the sweet potatoes in a pan of boiling water and cook for 15 minutes. Remove and let cool slightly before peeling and slicing.
Pour the chicken mixture into an ovenproof dish and top with the sweet potato slices. Brush with the remaining 1 tablespoon olive oil and cook in the oven for 30 minutes.