Heat vegetable oil or ghee in a skillet over medium heat. Add onion and sauté for about 5 minutes until translucent. Stir in garlic and ginger, cooking for an additional 1-2 minutes.

Incorporate garam masala, cumin, coriander, turmeric, paprika, salt, and cayenne into the onion mixture. Stir for about 2 minutes.

Add chicken thighs to the skillet, stirring to coat in the spice mixture. Cook for about 5 minutes until browned.

Transfer the chicken and spice mixture to the slow cooker, scraping any remaining spices from the skillet.

Add crushed tomatoes, coconut milk, and tomato paste to the slow cooker, stirring until combined.

Cover and cook on low for 6-8 hours or high for 3-4 hours until chicken is tender.

Stir in lemon juice before serving. Taste and adjust seasoning if necessary.

Serve hot, garnished with fresh cilantro and alongside basmati rice or naan.