Step 1: Add the Chicken - Place the cubed chicken breast into the bottom of your slow cooker. I like to spread it out a bit so the pieces are not all piled in one tight mound. You do not need to brown the chicken first for this easy slow cooker chicken tikka masala, which is part of what makes it so useful on a busy day. The chicken will cook gently in the sauce and come out tender enough to spoon right over rice.
Step 2: Whisk the Sauce - In a medium mixing bowl, whisk together the tomato puree, plain yogurt, salt, pepper, garam masala, paprika, turmeric, cumin, granulated sugar, ginger paste, and minced garlic. Take a minute here and whisk until the yogurt is fully blended into the tomato puree. That gives you a smoother sauce and helps the spices spread evenly. The sauce should look rich, fragrant, and a little orange from all those warm spices.
Step 3: Pour the Sauce Over the Chicken - Pour the sauce right over the chicken in the slow cooker. Use a spoon or spatula to scrape every last bit from the bowl, because that is where the flavor lives. The sauce may look a little thick at this point, and that is fine. As the chicken cooks, it releases some juices, the onion softens, and everything settles into a beautiful creamy tomato curry sauce.
Step 4: Stir in the Onion - Add the diced yellow onion to the slow cooker and stir everything together until the chicken is coated. This is a simple step, but it matters. You want each piece of chicken tucked into the sauce so it can pick up the flavor while it cooks. The onion will mellow as it cooks and bring a little sweetness that balances the tomato and spices nicely.
Step 5: Slow Cook Until Tender - Cover the slow cooker and cook on high for 3 to 4 hours or on low for 6 to 8 hours. I usually choose low if I am starting it earlier in the day and high if dinner is sneaking up on me. The chicken is done when it is cooked through and tender. Give it a gentle stir near the end if you are around, but do not fuss over it. The slow cooker knows its job.
Step 6: Finish With Cream and Cilantro - When the chicken is finished cooking, stir in the heavy cream. This is the moment the sauce turns silky and rich, so do not skip it unless you need a lighter version. Let it sit for a few minutes with the heat off or on warm so the cream blends in nicely. Spoon the chicken tikka masala over basmati rice, top with cilantro as desired, and serve with naan if you have it. Around here, naan is basically a sauce delivery tool, and I respect that.