In a bowl or directly in the slow cooker, combine the Greek yogurt, salt, garam masala, cumin, ground ginger, turmeric, and minced garlic to make the marinade.
Add the cubed chicken and diced onion to the marinade and stir well to coat all pieces evenly.
Spread the coated chicken and onion mixture evenly across the bottom of the slow cooker.
Pour the tomato sauce over the chicken, cover with the lid, and cook on high for 3 to 3½ hours, until the chicken is cooked through and tender.
Stir the contents of the slow cooker, then pour in the heavy cream and stir until combined and warmed through.
Serve the tikka masala over rice or with naan and garnish with chopped cilantro if desired.