Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a large frying pan, add the onion and fry until softened. Stir in the red pepper, tomatoes and garlic and fry for 1–2 minutes.
Pour in the stock and add the tomato purée, sugar, vinegar and a little salt and pepper and bring to the boil, stirring. Pour into the slow cooker pot, cover with the lid and cook on high for 2½–3 hours or until the vegetables are tender.
Purée the soup while still in the slow cooker pot with a stick blender. Alternatively, transfer to a liquidizer and purée, in batches if necessary, until smooth, then return to the slow cooker pot and reheat on high for 15 minutes.
Taste and adjust the seasoning, if needed, then ladle the soup into bowls and garnish with a drizzle of extra vinegar or stir in spoonfuls of spring onion pesto (see below).