Crumble one slice of naan and place in bottom of slow cooker
Add the red onion, potatoes, serrano pepper and chopped cilantro stems.
Arrange chicken thighs in a single layer over vegetables
Sprinkle chicken thighs with garam masala, kosher salt and black pepper.
Dot with butter. Add cinnamon stick.
Pour tomato puree over the top.
Cover and cook on low for 6 hours. Cooking too much longer will cause the potatoes to be overdone.
Turn slow cooker off. Uncover and gently stir in heavy cream. Let sit for about 10 minutes for sauce to thicken.
Serve chicken and sauce in shallow bowls. Top with chopped cilantro leaves, a slice of lime and wedges of warm naan bread.