Heat the olive oil and butter in the insert pan of a slow cooker or a frying pan over medium heat. Add the onion and garlic and cook, stirring, for 5–7 minutes until softened.
Return the insert pan, if using, to the slow cooker, or transfer the onion mixture to the slow cooker. Add the rice, stirring to coat the grains in the oil, then add the stock and cinnamon stick. Cover and cook on