Prepare the ingredients by dicing the onion, mincing the garlic, and grating the ginger.
In the slow cooker, combine the diced onion, minced garlic, and grated ginger.
Add the chicken thighs on top of the aromatics.
Mix in the crushed tomatoes, coconut milk, yogurt, and butter.
Add the garam masala, ground cumin, turmeric, salt, and pepper to the slow cooker; stir to combine.
Cover and cook on low for 6-8 hours or high for 4 hours until chicken is tender and cooked through.
Once cooked, remove the chicken and shred it with two forks; return to the sauce.
Taste and adjust the seasoning if necessary before serving.
Serve hot over steamed rice or with naan, garnished with fresh cilantro.