Heat oil in a large non-stick frying pan over medium heat.
Fry vegetables in batches for 5 minutes until browned and season well.
Transfer vegetables to a slow cooker and stir in the flour.
Add tomatoes, purée, chilli, pesto, and stock, then stir again.
Cover and cook on high for 4 hours.
Prepare dumplings by seasoning flour in a mixing bowl.
Grate frozen butter into the flour in batches, tossing to coat.
Stir in cold water to form a soft dough.
Divide dough into 12 pieces and roll into balls.
Stir spinach into the hotpot until wilted, then place dumplings on top.
Cover and cook on high for 30 minutes until dumplings are fluffy.
Spoon hotpot onto plates and serve with dumplings.