Heat large saucepan over medium-high heat; add butter and onion. Cook and stir until onions are tender, about 2-3 minutes. Add garlic, cook until fragrant, 1 minute. Add garam marsala, curry powder, chili powder and ginger; cook and stir 1 minute more. Add flour. Drizzle in olive oil until a paste is formed. Whisk in coconut milk and tomato paste; cook and stir until combined and slightly thickened, 1-2 minutes. Season with salt and pepper., Using an immersion blender, carefully puree spice mixture. Transfer mixture into a 5-qt. slow cooker. Add chicken; gently stir to combine., Cover and cook on low until chicken is no longer pink, 3-4 hours. Serve with rice. If desired, sprinkle with cilantro and serve with naan bread.