Heat half the oil in a pan and brown beef pieces for 5 minutes.
Transfer meat to the slow cooker.
Fry onions in remaining oil for 5 minutes, then add garlic, chilli, and ginger for 3 minutes.
Stir in ground spices, fry for 1 minute, and transfer to the slow cooker.
Add chopped tomatoes and 400ml water to the slow cooker.
Stir contents, ensure everything is covered by liquid, and cook on low for 8 to 10 hours.
Stir in garam masala and yoghurt 30 minutes before serving.
Season with salt, extra chilli, and stir in fresh coriander to finish.