Preheat the oven to gas 6, 200°C, fan 180°C. Peel the onions, wash and trim the leeks, carrots and courgettes, deseed the peppers, then roughly chop into chunks.
Place a large roasting tin or casserole pan on a medium-high heat. Peel and bash the garlic, then add to the pan along with the oregano and 1 tbsp oil. Cook for 2 mins, then add all the veg and cook for 5 mins more. Transfer to the oven for 1 hr, or until soft and caramelised.
Carefully remove the pan from the oven and place back on the hob. Pour in the passata, then fill the empty cartons with water, swirl around, and stir it into the pan. Taste and season to perfection, then simmer gently for 5 mins or until thickened, stirring occasionally. Serve the sauce chunky or leave to cool a little, then blitz with a stick blender until smooth.