Begin by chopping the chicken thighs into bite-sized pieces, then season lightly with salt and pepper. Gather your minced garlic and grated ginger, and measure out the spices and tomato puree.
In a mixing bowl, combine the tomato puree with minced garlic, ginger, garam masala, and cumin. Stir until smooth and fragrant.
Layer the seasoned chicken pieces into the slow cooker, then pour the spice-infused tomato sauce over the top, ensuring all pieces are coated evenly.
Distribute the small cubes of butter over the chicken and sauce. Cover with the lid and set your slow cooker to low. Let it cook for 6 to 8 hours until the chicken is tender and the sauce is thickened and glossy.
Once cooking is complete, open the lid and gently stir the dish. If the sauce is too thin, leave the lid off and cook on high for an additional 15-20 minutes to reduce and thicken.
If desired, stir in the heavy cream or coconut milk for extra richness, then let it sit uncovered for 10 minutes to allow flavors to meld.
Garnish with freshly chopped cilantro for a bright, herbal finish. Serve hot over steamed rice or with warm naan for a cozy, flavorful meal.