Pat the chicken thighs dry with a paper towel and cut them into bite-sized, 1-inch pieces. In a medium bowl, toss the chicken pieces with the salt, black pepper, and turmeric powder until evenly coated. Set aside.

Heat the coconut oil in a large skillet or Dutch oven over medium-high heat. Add the chopped onion and cook, stirring occasionally, until softened and translucent, about 5-6 minutes.

Add the minced garlic and grated ginger to the skillet. Cook for about 1 minute until fragrant, stirring constantly to prevent burning.

Stir in the curry powder, ground cumin, ground coriander, and cayenne pepper (if using). Toast the spices for 30-60 seconds, stirring continuously, until they become very fragrant. This step is key to developing a deep flavor.

Add the seasoned chicken to the skillet. Cook for 5-7 minutes, stirring occasionally, until the chicken is lightly browned on all sides. It does not need to be cooked through at this point.

Pour in the coconut milk, undrained diced tomatoes, and chicken broth. Stir everything together, scraping up any browned bits from the bottom of the pan. Bring the mixture to a gentle simmer.

Once simmering, reduce the heat to low, cover the skillet, and let the curry cook for 15-20 minutes, or until the chicken is cooked through and tender, and the sauce has thickened slightly.

Remove the curry from the heat. Stir in the brown sugar and fresh lime juice. Taste and adjust seasoning if necessary, adding more salt, sugar, or lime juice as needed.

Garnish generously with fresh cilantro and serve hot over basmati rice or with naan bread.