Mix chicken with yogurt, lemon juice, 1 teaspoon garam masala, paprika and 1/2 teaspoon salt. Refrigerate for 30 minutes; meanwhile, soak cashews in the measured boiling water for 20 minutes.
Heat oil in a skillet over medium heat and cook onion for 8 minutes. Add garlic, ginger and remaining garam masala for 1 minute, then add tomato sauce and simmer for 3 minutes.
Blend the tomato mixture with cashews, their soaking water and remaining salt until smooth, venting steam carefully. Pour into a 5-quart slow cooker.
Heat the broiler. Spread chicken on a foil-lined rimmed pan and broil 6 inches from the heat for 6 to 8 minutes, turning once, until a few edges color; the chicken is not fully cooked yet. Transfer chicken and its juices directly to the hot sauce in the cooker.
Cover and cook on low for 5 to 6 hours until chicken reaches 165°F and is tender. Stir in butter, cream and crushed fenugreek, then cover for 10 minutes until the sauce is steaming.
Warm naan according to its package directions during the final 10 minutes. Divide chicken and sauce among 6 bowls and serve one naan with each.