Preheat oven to 190°C. In a bowl mix together the breadcrumbs, Parmesan, Cheddar, garlic, basil and oregano, set aside.
In a pan, heat the oil and saute the onions over a moderate heat until soft. Griddle or grill the courgette slices for a few minutes on each side.
In a greased gratin dish, add the onions and tinned tomatoes and arrange the courgettes and tomatoes in layers on top with a coating of the cheese, bread and herb mixture between each layer. Top with the breadcrumb mixture and bake in the oven for 20 minutes until brown and crisp.