In a mixing bowl, combine the Greek yogurt, salt, garam masala, ground cumin, ground ginger, turmeric, and minced garlic until smooth.
Add the cubed chicken and diced onion to the bowl and toss to coat thoroughly in the yogurt-spice mixture.
Transfer the coated chicken and onion to the slow cooker and spread into an even layer.
Pour the tomato sauce evenly over the chicken mixture and close the slow cooker lid.
Cook on high for 3 to 3.5 hours (about 180–210 minutes), until the chicken is cooked through and tender.
Stir the contents of the slow cooker, then stir in the heavy cream until combined and heated through.
Serve the tikka masala over rice or with naan and garnish with chopped cilantro, if desired.
