Season the chicken thighs with salt and pepper. In a skillet over medium heat, add vegetable oil and brown the chicken on both sides for about 5 minutes. Remove from heat and set aside.
In the slow cooker, combine the chopped onion, garlic, ginger, garam masala, cumin, coriander, and turmeric. Stir to combine.
Add the browned chicken thighs on top of the spice mixture. Pour the tomato sauce and coconut milk over the chicken.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the chicken is tender and cooked through.
Once cooked, shred the chicken in the slow cooker using two forks. Stir in the butter and lemon juice until melted and well combined.
Serve hot, garnished with fresh cilantro, alongside cooked rice or naan.