Marinate the chicken: Combine all marinade ingredients in a large bowl. Add the chicken and toss to coat. Cover and refrigerate for at least 20 minutes and up to 8 hours.
Sear the chicken: Heat the oil and 2 tbsp butter in a Dutch oven over medium-high heat. Add the chicken in a single layer (work in batches if needed) and sear 2-3 minutes per side until lightly charred. It does not need to cook through. Set aside.
Build the base: In the same pot over medium heat, add the onion and cook 5-6 minutes until soft and golden at the edges. Stir in the garlic and ginger; cook 1 minute. Add the cumin, paprika, cardamom, and cayenne; stir 30 seconds until fragrant.
Simmer the sauce: Pour in the crushed tomatoes, cashews, water, sugar, and 1 tsp salt. Simmer on low, covered, 15 minutes until the cashews are completely soft.
Blend it silky: Blend until completely smooth (a full 60 seconds in a countertop blender, or use an immersion blender).
Finish: Return the sauce to the pot over low heat. Stir in the cream, remaining 2 tbsp butter, and fenugreek leaves. Add the chicken and any juices. Simmer 5-7 minutes until cooked through. Adjust salt and cayenne.
Serve over basmati rice or with warm naan; garnish with cilantro and a drizzle of cream.