In a large bowl, combine the chicken, onion, garlic, ginger, tomato puree, yogurt, olive oil, garam masala, turmeric, cumin, paprika, salt, cinnamon, black pepper, and cayenne pepper. Stir thoroughly to ensure the chicken is evenly coated with the mixture.
Cover the bowl and refrigerate for at least two hours, or preferably overnight, to allow the flavors to marinate.
Transfer the marinated chicken mixture into the slow cooker, then add the bay leaves on top.
Set the slow cooker to low and cook for 4 hours (or 2 hours on high for faster cooking).
When the cooking time is done, whisk the heavy cream and cornstarch together in a medium bowl until smooth, then stir this mixture into the slow cooker. Let the tikka masala cook for another 20 minutes to thicken.
Squeeze the lemon juice over the dish, stir to incorporate, and remove the bay leaves.
Serve the chicken tikka masala hot, garnished with fresh parsley or cilantro, over a bed of white or brown rice.