Combine the roughly chopped red onions, garlic cloves, fresh ginger, tomato purée, along with all the spices—garam masala, ground cumin, turmeric, paprika, cayenne pepper—and salt in a mini food chopper. Add the chicken stock to help blend everything smoothly. Process until you achieve a luscious, smooth paste.

Place your skinless and boneless chicken into the slow cooker and pour in the curry paste. Mix well until each piece of chicken is coated in the mixture.

Set your slow cooker to high heat for 3 hours or low heat for 5 hours. Avoid lifting the lid during cooking.

Stir in heavy cream or fat-free natural yogurt right before serving. Allow it to heat through for a few minutes.

Taste your curry and adjust seasoning if needed. Garnish with fresh cilantro and red pepper flakes or onion seeds before serving.