01 - Combine yogurt, lemon juice, turmeric, chili powder, and salt in a large bowl. Add chicken thighs and toss thoroughly to coat. Marinate for at least 30 minutes or up to overnight in the refrigerator.
02 - Heat a skillet over medium heat with a splash of oil. Cook sliced onions until golden brown, approximately 8 minutes. Add minced garlic and grated ginger, cooking for an additional minute.
03 - Incorporate diced tomatoes, tomato paste, cumin, coriander, garam masala, paprika, cinnamon, and bay leaves into the skillet. Cook while stirring until fragrant, about 3 minutes.
04 - Move the onion and spice mixture into a slow cooker.
05 - Place the marinated chicken atop the mixture in the slow cooker. Pour in coconut milk or chicken stock and gently stir to combine.
06 - Cover and cook on low for 4 hours until the chicken is tender and the sauce has thickened.
07 - Remove bay leaves and adjust salt seasoning as needed.
08 - Sprinkle fresh cilantro leaves and sliced chilies over the curry before serving.