Heat 3 tablespoons olive oil in a pan, add the onions and courgettes and fry for 10 minutes, stirring occasionally. Add the garlic, tomatoes, tomato puree and salt and pepper to taste. Cover and simmer for 5 minutes, then turn into a 1.5 litre (2 1/2 pint) ovenproof dish.
For the topping beat the potatoes with the spring onions, oil, milk and salt and pepper to taste. Spoon over the courgette mixture to cover.
Bake in a preheated for 30-40 minutes until golden.
You can easily make this a vegan pie by using unsweetened soya milk instead of regular milk in the mash.