Place the onion, garlic and chilli in a blender and blend to a paste. Add the tomatoes and blend again.
With the motor running add the olive oil in a light, steady stream, to form an emulsion.
Season generously with salt and black pepper. Place in a saucepan and bring to the boil. Reduce to a simmer and allow the sauce to cook for 10-15 minutes.
Finally stir in the basil.
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