Brown onion, garlic and ginger in the oil.
When browned, add the spices except the garam masala and stir fry for a couple of minutes.
Add the passata.
Bring to the boil then reduce to a simmer. Add the garam masala.
Add the spinach and let it wilt. Stir to mix it in once wilted.
Pour the mixture into your slow cooker.
Add the chopped chicken and combine with the sauce.
Cook on low for 6-7 hours.
Half an hour before the end, taste and add more garam masala if needed.
If you like, pull the chicken apart and stir into the sauce. This helps make the chicken go a bit further I find.
Serve with rice, poppadoms or naan bread and raita.