Combine Base Ingredients: In the basin of your slow cooker, place the chopped onion, minced garlic, and grated ginger. Add the olive oil, plain Greek yogurt, and tomato puree. Stir these ingredients together until well combined.
Add Spices: Gently mix in the Garam Masala, ground cumin, salt, paprika, cayenne pepper, ground black pepper, ground cinnamon, and ground turmeric. Ensure all spices are evenly distributed throughout the sauce base.
Incorporate Chicken & Cook: Add the cubed chicken pieces to the slow cooker. Stir thoroughly to ensure each piece of chicken is fully coated with the aromatic sauce. Secure the lid on the slow cooker. Cook on the LOW setting for 5-6 hours, or on the HIGH setting for 3-4 hours, or until the chicken is tender and cooked through.
Prepare Thickener: Approximately 20-30 minutes before the cooking cycle is complete, whisk the cornstarch into the heavy cream in a small bowl until smooth and no lumps remain.
Finish the Sauce: Gently stir the cornstarch-cream slurry into the sauce in the slow cooker. Replace the lid and continue to cook for the remaining 20-30 minutes. This will allow the sauce to meld with the cream and thicken to a rich, velvety consistency.
Serve: Once ready, ladle the Slow Cooker Crimson Chicken Tikka into bowls. Garnish with fresh chopped cilantro, if desired. See notes below for serving suggestions.