Pat the chicken thighs dry with paper towels.
Season the chicken with salt, black pepper, paprika, and cumin.
Melt the butter in a skillet over medium heat.
Add the chopped onion and cook for 4–5 minutes, until softened.
Add the garlic and fresh ginger and cook for another 30–60 seconds.
Add garam masala, cumin, coriander, paprika, turmeric, and chili powder if using.
Stir the spices for about 30 seconds to release their aromas.
Add the crushed tomatoes and tomato paste.
Stir in the chicken broth and optional brown sugar.
Place the seasoned chicken in the slow cooker.
Pour the tomato sauce over the chicken and stir gently to coat.
Cover and cook on LOW for 5–6 hours or HIGH for 2½–3½ hours.
Check that the chicken is fully cooked and tender.
About 20–30 minutes before serving, stir in the heavy cream.
Taste the sauce and adjust salt and spices as needed.
Add an extra tablespoon of butter for a richer sauce if desired.
Allow the sauce to heat through without bringing it to a vigorous boil.
Remove the chicken and serve over basmati rice.
Spoon plenty of creamy sauce over the chicken.
Garnish with fresh cilantro and lime wedges.
Serve with warm naan.