Heat 3 tablespoons neutral oil in a saucepot over medium-low heat. Add the cardamom pods, broken cinnamon, cracked coriander seeds, cloves, and cracked peppercorns and toast, stirring, for 3–4 minutes until aromatic.
Optional: remove the whole toasted spices and grind them in a mortar and pestle, then return the ground spices to the pot if you prefer no large pieces in the sauce.
Add the sliced red onions to the spiced oil and cook, stirring frequently, until very soft and dark brown, about 30 minutes.
Stir in the minced garlic and ginger and cook for 2–3 minutes until fragrant.
Add the turmeric, ground coriander, ground cumin, and chili powder and cook over low heat for 2–3 minutes, stirring to toast the spices briefly.
Pour in the chopped (or roughly pureed) tomatoes, bring to a simmer, and cook over medium heat, stirring occasionally, until the mixture reduces and becomes thick like a paste, about 20–25 minutes. Season the sauce with coarse salt to taste.
Meanwhile, heat 3 tablespoons neutral oil in a large frying pan over high heat until lightly smoking. Add the cubed chicken in a single layer and cook, turning as needed, 7–9 minutes until lightly browned and cooked through. Season the chicken with coarse salt.
Add the cooked chicken to the tomato-spice sauce, stir to combine, and simmer over medium heat for a few minutes to meld flavors.
Stir in the minced fresh cilantro, adjust seasoning, and serve.