In a large mixing bowl combine chopped onions, minced garlic, grated ginger, tomato puree, plain yogurt, olive oil, lemon juice, Garam Masala, cumin, paprika, salt, cinnamon, pepper and cayenne pepper.
Stir until combine. Pour half of sauce mixture into a large slow cooker then add in diced chicken followed then cover chicken with remaining sauce. Add in 2 bay leaves. Cover slow cooker with lid and cook on low heat for 6 - 8 hours (or high for 3 - 4 hours).
In a mixing bowl, whisk together heavy cream and cornstarch, pour mixture into slow cooker and gently stir. Allow mixture to cook 20 minutes while you prepare the rice. Remove bay leaves and serve warm over rice and sprinkle with chopped cilantro.
*I put 1 -3 tsp because it is all depending on your heat preference. If you don't like spicy foods you could even omit this ingredient. I go for 3 tsp but I like my food really spicy.