Grease the inside of your slow cooker.
Heat a frying pan to a medium heat, add the cumin seeds and gently toast for about a minute.
Add 2 tablespoons of oil to frying pan and fry sliced onion for 5 minutes before transferring to the slow cooker.
Add a tablespoon of oil to the frying pan, maintaining a medium heat.
Add the ground coriander, turmeric, chilli flakes, fenugreek seeds, Kashmiri chilli powder, salt, pepper, tamarind paste, green chillies, tomato paste, garlic and ginger. Cook through for two minutes.
Add the chicken pieces, coat each piece in the spice mix and fry for about five minutes. Transfer the chicken to the slow cooker.
Add passata and water to your slow cooker and give the jalfrezi a good stir.
Cook on high for 2 ½ hours or low for 4 ½ hours.
Add 1 teaspoon of garam masala along with the sliced red and green bell peppers. Stir well and cook on high for an hour.
Stir in the freshly chopped coriander before serving with lemon wedges and rice or naan breads.