Heat the olive oil in a skillet or a slow cooker with a sauté function. Note: this step is optional. You can add the onion in raw and omit the olive oil in the recipe.
Add the diced onion and cook until softened, about 5 minutes.
In a bowl, mix the coconut milk, tomato paste, ginger, garlic, curry powder, garam masala, chili powder, and salt.
Place the chicken thighs and onion in the slow cooker and pour the sauce over the top.
Top the chicken with the butter.
Cover and cook on high for 3 to 4 hours or on low for 6 to 7 hours.
Remove the chicken, shred it, and stir it back into the sauce.
Serve with naan and/or rice. Sprinkle with chopped cilantro .Enjoy!