Heat the rapeseed oil in a 3 qt balti dish or large Dutch oven over medium heat.
Add the chopped onion and cook, stirring occasionally, until softened and beginning to brown, about 5 minutes.
Add the sliced garlic and sliced ginger, season lightly with salt, and cook until fragrant, about 30 seconds.
Add the chopped tomatoes and cook for 3 minutes, stirring occasionally to break them down.
Stir in the Kashmiri chili powder, turmeric, garam masala, ground coriander, fenugreek seeds, and chopped green chilies; cook and stir for 1 minute to bloom the spices.
Add the cubed chicken and cook, stirring occasionally, until the exterior is no longer pink, about 4–5 minutes.
Pour in the cold water, bring the mixture to a boil, then reduce heat to a simmer and cook until the sauce thickens and the chicken is cooked through, about 5–10 minutes.
Taste and adjust seasoning with salt as needed, then remove from heat.
Garnish with the chopped cilantro and serve hot over basmati rice with naan if desired.
