Start by dicing one medium onion, mincing four cloves of garlic, and grating about one tablespoon of fresh ginger. Trim any excess fat from the chicken thighs and pat them dry with paper towels.
Heat two tablespoons of olive oil in a skillet over medium heat. Add the diced onion and cook until translucent, about 5-7 minutes. Stir in the garlic, ginger, 3 tablespoons of tikka masala curry powder, 1 teaspoon each of cumin, coriander, turmeric, and garam masala. Sauté for another 1-2 minutes until the spices become fragrant.
Transfer the sautéed mixture to your slow cooker. Add the two cans of diced tomatoes, 1 cup of coconut milk, 2 tablespoons of tomato paste, and season with salt and pepper. Stir well to combine.
Place the chicken thighs into the slow cooker, nestling them into the sauce. Ensure they are mostly submerged for even cooking.
Set your slow cooker to low and cook for 6 to 8 hours, or on high for 3 to 4 hours. The chicken should be tender and the sauce thickened.
Before serving, taste and adjust the seasoning with additional salt or pepper if needed. Garnish with freshly chopped cilantro for a burst of color and freshness.
Serve your Slow Cooker Chicken Tikka Masala hot over fluffy cooked rice. This dish pairs beautifully with naan or steamed vegetables for a complete meal.