Place tomatoes, garlic, onion, chilli, celery and capsicum in a 4.5-litre (18-cup) slow cooker. Cook, covered, on low, for 7 hours.
2.
Cool soup for 10 minutes. Using a stick blender, blend soup until smooth. Season to taste.
3.
Make basil dumpling mixture: Place flour in a medium bowl; using your fingertips, rub in butter. Stir in egg, pesto and enough of the milk to make a soft, sticky dough.
4.
Carefully drop level tablespoons of the basil dumpling mixture, about 2cm (ยพ-inch) apart, on top of soup. Cook, covered, on low, for 1 hour or until dumplings are firm to touch and cooked through.
5.
Serve soup with dumplings and sprinkled with basil. Top with halve mixed baby tomatoes, if you like.