Add onion, garlic, ginger, crushed tomatoes, olive oil, garam masala, cumin, paprika, and salt to the slow cooker. Stir to combine.
Add the chicken and turn to coat evenly in the sauce.
Cover and cook on LOW for 6–7 hours or HIGH for 3–4 hours, until the chicken is tender and fully cooked.
Reduce heat to LOW. Stir in butter and heavy cream until the sauce is smooth and creamy.
Let cook for an additional 10–15 minutes to warm through.
Taste and adjust seasoning if needed. Garnish with cilantro before serving.