Preheat the oven to 200°C.
Heat oil in a saucepan and fry onion and sweet potato for 5 minutes until soft.
Add garlic and fry for 2 minutes.
Add sauce ingredients and 500ml water, then simmer for 20 minutes until vegetables are soft.
Soak bread in milk in a large bowl for a few minutes.
Mix bread with onion, carrot, courgette, mince, and egg yolk, then season with pepper.
Roll into 24 balls and place in a 2 litre ovenproof dish.
Bake meatballs for 20 minutes then drain any excess liquid.
Remove bay leaf and blend the sauce until smooth.
Pour sauce over meatballs, sprinkle with cheese, and bake for 25 minutes until bubbling.