Combine the chilli powder, cumin, paprika, coriander and oregano with a good pinch of salt and freshly ground black pepper. Coat the chicken breasts in the spices and tip into a slow cooker with any leftover spice mix.

Scatter over the peppers and onions. Combine the tomato purée with 2 tbsp water, then pour into the slow cooker. Cook on high for 3 hrs or low for 6 hrs until the chicken is cooked through and the peppers and onion have softened. Remove the chicken and slice, then mix back in. Serve in tortillas with guacamole, soured cream and salsa, if you like.