Heat the butter in a saucepan over a medium heat, then once it's melted add the onions.
Cook the onions until soft, then pour in the can of tomatoes and the cherry tomatoes.
Bring the pan to the boil, then reduce the heat to simmer for 8-10 minutes
Using a stick blender, smooth out the soup
Stir in the double cream
Season with salt & pepper to taste and sprinkle on the dried basil.