In a large skillet over medium heat, melt 2 tbsp butter. Add the roughly chopped onions and cook, stirring occasionally, until caramelized and golden brown (~10-15 minutes).

Stir in the 3 tbsp tomato paste, 1 tbsp garlic paste, and 1 tbsp ginger paste. Cook for 1-2 minutes until fragrant.

Add the spices: 2 tsp paprika, 1 tsp cayenne, 2 tsp garam masala, 1 tsp cumin, 1 tsp coriander, 1 tsp salt, and ½ tsp black pepper. Toast for 30 seconds to bloom the flavors.

Deglaze the pan by pouring in the 14.5 oz can fire-roasted tomatoes (with juices) and ½ cup water. Scrape up any browned bits from the bottom. Simmer for 2-3 minutes.

Transfer the mixture to a blender (or use an immersion blender) and blend on high for about 2 minutes until completely smooth and creamy. (If it's too thick, add a splash more water.)

Place the 2 lbs diced chicken in the bottom of a 7-quart slow cooker. Pour the blended sauce over the top. Stir gently to coat.

Cover and cook on High for 2-3 hours or Low for 3-4 hours, until the chicken is tender and shreds easily with a fork.

Stir in the remaining 2 tbsp butter until melted.

Gently fold in the 1 cup Greek yogurt until fully incorporated (this adds creaminess without curdling—avoid boiling after this step).

Cover and let sit on Warm for another 10-15 minutes to meld the flavors.

Meanwhile, rinse the 1½ cups basmati rice under cold water. In a pot, bring 3 cups water to a boil, add the rice, reduce to a simmer, cover, and cook for 12-15 minutes until fluffy. Fluff with a fork.

Divide the rice evenly across 7 meal prep containers (~¼ cup cooked rice per serving). Top each with ~1 cup of butter chicken. (Optional: Garnish with fresh cilantro and serve with naan on the side.)