Heat the oil in a medium sized frying pan over medium heat until the oil just starts to shimmer.
Stir in the garlic ginger paste. Gently fry until the garlic ginger paste stops sputtering. This can get a little messy.
Add the chopped green chilies. Cook for about 30 seconds, stirring constantly.
Turn your heat down to medium low and add your spice mix. Cook for about 30 seconds. You want to fry your spices in the oil. Do not skimp on the oil. Bad things happen if the spices stick and burn. The oil (and temperature control) are your friends. Nothing worse than burned spices. Chuck it and start again if that happens. There's no fixing it...
Add the Indian hotel curry gravy. Stir it really well to get the oil to combine with the curry gravy. You want everything mixed together at this point. Bring to a simmer.
Stir in the cilantro.
Add the chicken thigh pieces in a single layer. Nestle them down into the sauce. Cover and cook about 5 minutes.
Remove the lid, flip the chicken and replace the lid. Continue simmering until the chicken is done. You are shooting for an internal temperature of 160F. It will get to 170F as the curry finishes cooking. Don't have an instant read thermometer? Not a bad idea to get one. Done is not subjective. It's a measurement that you cannot do without a thermometer. You are guessing otherwise.
Add the spinach and stir it to combine.
At this point you have a decision to make. If the chicken threw a lot of liquid the sauce might be right. My guess is you'd like it a bit thinner. More like a restaurant sauce. Add a couple tablespoons of chicken stock. Check it again. If it's still too thick, add a bit more stock. I've never added more than a 1/4 cup.
Add the lemon juice, cover, and simmer for another two minutes.
Palak chicken goes great with chapatis or rice. A nice tarka dal rounds the meal out nicely.