Heat the oven to 190°C (170° fan) gas 5.
Put the potatoes in a pan with water to cover, bring to a boil and simmer for 3 minutes.
Brush1 tablespoon of oil over the base and sides of a baking dish.
Layer the potatoes, tomatoes, onion and rosemary in the dish, seasoning and drizzling each layer with oil.
Bake for 45-55 minutes, until the top, is lightly browned.