Heat ¼ cup oil in a large pan or skillet over medium-high heat. Add chopped onions and sauté for around 10 minutes.
Pour the onions and oil into a blender. Add minced garlic, grated ginger, crushed tomatoes, and water. Blitz to a puree.
Mix together ground coriander, ground cumin, cayenne pepper, sweet paprika, ground fenugreek powder, black pepper, turmeric, and salt in a bowl.
Peel and roughly chop one more onion and cut chicken thighs into bite-sized pieces.
Heat the remaining 2 tablespoons of oil in the same pan until shimmering. Fry the chicken for about 5 minutes.
Add in the chopped onions and cook for 2 minutes to get the flavors blending together.
Add the remaining oil and grated ginger, sprinkle over the spice blend, and cook for 2 more minutes.
Incorporate the curry base from the blender, bring to a simmer, and cook for 15 minutes.
Bring the curry to a boil and cook over high heat, stirring often until the sauce starts to separate, about 7 to 10 minutes.
Finish with chopped green chilies and cilantro before serving.
