This is one of those dishes that benefits from having all your ingredients measured out and ready to go before you start cooking. I like to do this while the chicken is marinating.
You can control the heat in this curry by adjusting the amount of Kashmiri chili powder or cayenne you put into the dish. When in doubt, use less and add a bit more at the end, if needed.
Marinate the chicken: Place the cubed chicken in a bowl. Add the chili powder and lime juice and toss to combine well. Cover and let stand while you prepare the other ingredients and start cooking. (or make ahead, cover and refrigerate for up to 2 hours).
For the sauce: Heat the oil in a large pan over medium heat. Add the onion and cook, stirring, until very soft, 3-4 minutes. Add the garlic and cook briefly, then add the ginger paste and cook 30 seconds more. Add the green chili or jalapeno and spices (cumin, coriander, turmeric, Kashmiri chili powder, garam masala, salt and pepper) and cook, stirring, for an additional 30 seconds. Add the tomato paste and stir into a paste, cooking 30-45 seconds.
Add the marinated chicken cubes to the pan and cook, stirring, until chicken is no longer pink. Add the passata, chopped tomatoes, chopped red bell pepper and dried fenugreek leaves.
Bring up to a boil, then reduce heat and allow to simmer and cook for 15 minutes.
Stir in the cilantro to combine, then scatter the spinach over-top and cook, stirring, until the spinach wilts down.
Taste the curry and add additional salt, as needed.
Notes
Be sure to use Indian chili powder and not the kind of chili powder that you would use for a pot of chili. They are very different blends of spices.Be sure to read the Ingredient Notes above this recipe card, for more details on the ingredients, as well as step-by-step photos that you might find useful.