Add oil to a large skillet or saucepan and cook onion and garlic until soft and fragrant, about 5 minutes.
Add fresh ginger, turmeric, garam masala, cumin, chili powder, salt, pepper, cinnamon and cayenne and toss to combine. Cook for another 1-2 minutes. This step can be done in the base of your slow cooker if it allows.
Transfer onion and garlic mixture into your slow cooker. Add chicken, coconut milk, tomato sauce, lemon juice and cinnamon stick to the slow cooker as well.
Cover and cook on high for 3 hours or low for 6 hours. Add green beans when there's about 1 hour left in the cooking process.
Serve butter chicken over cauliflower rice (or regular rice) with fresh cilantro and a wedge of lemon for squeezing.
