Place the defrosted spinach in a blender and puree until fairly smooth, using 1-2 tablespoons of water to loosen up the mixture. Set aside.
Heat the oil in a large nonstick skillet set over medium heat.
Add the onions and cook, stirring occasionally, until the onions start to brown, 8 to 10 minutes.
Stir in the crushed tomatoes, garlic paste, ginger paste, coriander, cayenne pepper and turmeric, and cook for 1 minute.
Add the chicken, stirring to coat with the tomato mixture. Simmer, stirring occasionally, until the chicken is almost cooked through, 10 to 12 minutes.
Stir in the pureed spinach and garam masala. Simmer for 5 minutes.