In a large pan, heat the vegetable oil over medium heat. Once hot, add the finely sliced onions and sauté them until they turn a rich golden brown, stirring occasionally to prevent burning.
Stir in the minced garlic and grated ginger, allowing them to cook together for about 1 minute, or until fragrant and lightly golden.
Incorporate the pureed tomatoes into the pan along with the ginger-garlic paste. Cook this mixture on medium heat until the oil begins to separate from the masala, which should take around 5-7 minutes.
Add the ground coriander, ground cumin, turmeric powder, garam masala, red chili powder, and a pinch of salt. Stir well, letting the spices infuse into the masala for another 2-3 minutes.
Introduce the chicken pieces to the pan, stirring well to ensure they are thoroughly coated with the vibrant spice mixture. Cook the chicken for about 5 minutes, or until the pieces are starting to brown slightly.
Pour in the coconut milk, stirring gently to combine everything beautifully. Allow the mixture to come to a gentle simmer.
Cover the pan with a lid and let it cook on low heat for approximately 20-25 minutes. This slow cooking will ensure that the chicken is fully cooked through and tender. Remember to stir occasionally to prevent any sticking at the bottom.
Taste and adjust the seasoning with additional salt, then add the freshly squeezed lemon juice and the slit green chilies if you like some extra heat. Stir well to combine.
Remove the pan from heat, and lavishly garnish with fresh cilantro leaves right before serving.
Notes
Serve hot with naan or basmati rice for a complete meal.