Combine yogurt, lemon juice, half the garlic and ginger, turmeric, and 1/2 tbsp garam masala. Toss chicken in marinade and refrigerate for at least 1 hour, preferably overnight.

Heat 1 tbsp butter in a skillet and brown chicken 1–2 minutes per side for extra color. This step is optional.

Melt 2 tbsp butter in a pan and cook chopped onion until soft. Add remaining garlic and ginger and cook 1 minute more.

Place onions and chicken in the slow cooker. Add tomato sauce, tomato paste, cumin, remaining garam masala, chili powder, brown sugar, salt, and 1/2 cup water. Cook on LOW 4–6 hours or HIGH 2–3 hours.

Remove chicken, shred or chop, then return to the crockpot. Stir in remaining butter, heavy cream (or coconut milk), and fenugreek if using. Adjust seasoning and simmer uncovered 15–30 minutes if you want a thicker sauce.

Garnish with chopped cilantro and a squeeze of lemon. Serve over basmati rice or with warm naan.