Heat the olive oil over medium heat in a large saucepan. Sauté the red onion, garlic and red capsicum with a pinch of salt over medium heat until soft, about 5 minutes.
Add the chicken stock and tinned cherry tomatoes and bring to a boil, then turn down to a simmer. Cook for ten minutes, stirring often.
Remove from heat. Allow to cool, add the basil leaves (reserve a few for the garnish), then puree with a hand-held blender. Season to taste.
Refrigerate for about 3 hours. When ready to serve, stir in the balsamic vinegar, then garnish with basil leaves.
The soup will keep in a sealed container in the fridge for up to 5 days. It is also suitable to freeze.