Heat a pan over a medium heat and add the oil or a couple tablespons of water.
When hot add the onions and garlic and cook for a couple of minutes until just starting to turn golden.
Add the chopped tomatoes. Continue to cook over a medium heat, stirring every few minutes until the tomatoes have broken down and are soft.
Remove from the heat and add basil and salt and pepper then blitz in a blender or with a stick immersion blender until smooth.
I have tried this recipe with large tomatoes and grape/cherry tomatoes and it works well with both, or even a mixture of the two.Leftovers will keep int he fridge for 4 to 5 days and reheat really well in a pan or in a microwave.