Heat oil in a large saucepan over medium heat. Stir in cumin seed and cook until they start to pop, 20 to 45 seconds. Stir in onion and cook until golden brown, about 5 minutes. Season with turmeric, cayenne, garam masala, garlic and ginger. Cook for 1 to 2 minutes until fragrant.
Puree the mixture with the tomatoes in a blender until smooth. Return the puree to the saucepan and add the chicken. Simmer gently until the chicken has cooked, about 20 minutes; add water as needed while cooking to maintain desired consistency.