1. In a slow cooker, add the chicken, onion, garlic, and ginger.
2. In a separate bowl, mix together the diced tomatoes, coconut milk, tomato paste, olive oil, turmeric, cumin, coriander, paprika, garam masala, salt, and pepper.
3. Pour the mixture over the chicken and stir to coat evenly.
4. Cover the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours.
5. Once the chicken is cooked, taste and adjust seasoning if needed.
6. Serve the chicken tikka masala over cooked rice, if desired.
7. Garnish with fresh cilantro, if desired.
8. Enjoy your delicious and easy slow cooker chicken tikka masala!