Preheat the slow cooker if necessary; see the manufacturer's instructions. Arrange the chicken, onion and sweet potatoes in the base of the slow cooker pot in an even layer. Sprinkle with the garlic.
Bring the curry sauce just to the boil in a small saucepan or the microwave. Pour into the slow cooker pot in an even layer. Cover with the lid and cook on high for 30 minutes. Reduce the heat and cook on low for 5½–6½ hours, or set to auto for 6–7 hours, until the chicken is cooked through and the sauce piping hot.
Stir well, then sprinkle with roughly chopped coriander, if liked. Spoon into bowls and serve with warmed naan bread.