In a bowl, combine all the ingredients for the chicken marinade: yogurt, lemon juice, cumin, coriander, paprika, turmeric, cinnamon, cayenne pepper, minced garlic, minced ginger, salt, and black pepper. Mix well.
Add the chicken pieces to the marinade, ensuring they are well-coated. Cover and refrigerate for at least 1 hour, or preferably overnight for the best flavor.
In a large skillet, heat the olive oil over medium heat. Add the chopped onions and sauté until they become soft and translucent, about 5-7 minutes.
Add the minced garlic and sauté for another 1-2 minutes until fragrant.
Stir in the cumin, coriander, paprika, turmeric, cinnamon, and cayenne pepper. Cook for 1-2 minutes to toast the spices and release their flavors.
Transfer the onion and spice mixture to your slow cooker.
Add the marinated chicken and tomatoes to the slow cooker. Stir to combine everything.
Cover and cook on LOW for 4-6 hours or on HIGH for 2-3 hours until the chicken is tender and fully cooked.
About 30 minutes before serving, stir in the half and half. Season with salt and black pepper to taste. Allow the sauce to heat through.
Serve your slow cooker chicken tikka masala over cooked rice or with naan bread. Garnish with chopped fresh cilantro if desired.
This makes 6 servings. On Weight Watchers, each serving is 2 WW Points. Enjoy!