Heat the oil over a high heat in a large, nonstick frying pan, add the onion and fry, stirring, for 5 minutes or until lightly browned. Add the garlic and remaining fresh vegetables and fry for a further 2 minutes.
Stir in the tomatoes, stock, sugar and a little seasoning. Bring to the boil, stirring, then transfer to a deep ovenproof dish. Cover the top of the dish with foil and bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 45—60 minutes until the vegetables are tender.
When the vegetables are almost ready, grill one side of the bread slices. Slice the cheese and add one slice to each untoasted side of bread. Remove the foil from the ratatouille, stir the vegetables and top with the toasts, cheese side up.
Put the ratatouille under a preheated hot grill for 4—5 minutes until the cheese is just beginning to melt. Spoon into shallow bowls and serve with a rocket salad.