Place the chicken, onion, garlic, ginger, garam masala, cumin, paprika, and turmeric in a slow cooker and stir to combine.
Add the diced tomatoes and stir to combine.
Add the heavy cream and stir to combine.
Cover and cook on low for 6-8 hours, or until the chicken is cooked through.
Stir the mixture and season with salt and pepper, to taste.
Garnish with fresh cilantro and serve with steamed rice or naan bread.