This isn't full blown Indian restaurant cooking but it's a lot closer than most recipes out there...
Place the onions in a microwave safe dish and cover loosely. Microwave at 70 percent until the onions are soft and translucent. This takes 10 minutes in my 1100 watt microwave. I can't predict how long it will take in yours...
Remove the onions from the microwave. Be careful. They will be hot. Let them cool slightly. Place the onions, 2 tbsp vegetable oil and 3/4 cup of warm water in a blender and puree until smooth. Set aside.
In a small bowl, combine the cumin, coriander, turmeric, garam masala, Kashmiri chili powder, kasoor methi and salt. This is your spice mix.
Heat 3 tbsp vegetable oil in a pot over medium heat until it shimmers.
Add the garlic ginger paste and cook until it stops spluttering.
Turn the heat to medium low. Add the spice mix and stir continuously until it starts to smell really good - around 30-40 seconds. Watch it carefully. If you burn the spices at this point you have to start over.
Add the diluted tomato paste and stir to combine. Turn the heat up to medium. Cook for 1 minute.
Add the onion paste and turn the heat up to medium high. Cook, stirring occasionally for 3-4 minutes. Don't worry if it seems a bit dry. The chicken will release juices as it cooks and you should be left with a nice amount of sauce.
Turn the heat down to medium low. Add the chicken and stir. Cover and simmer until the chicken is done. It's done when you measure the internal temperature and it says 170F, about 12-15 minutes. There's no other way to really tell when chicken is cooked. Get an instant read thermometer if you haven't already bought one. Just do it! That's what the pros do.
Add the thawed spinach and stir to distribute it. If the curry is a bit thick add a bit of water or chicken stock and stir. Let the chicken saag simmer another minute or so. Taste for salt and adjust as needed.