In a food processor or blender, combine yogurt, 4 cups water, gram flour, 2 tablespoons coriander, turmeric, and salt (do this in 2 batches because of the volume); set aside.
Make sure yogurt is thoroughly blended, or it will curdle during the cooking process.
Heat 2 tablespoons of the oil in a large wok or a saucepan over moderately high heat and cook, stirring, until the peppers sizzle, about 30-45 seconds (they may get white spots on them) (also, stand back in case peppers burst and may fly into your eyes).
Add the onion, ginger, and garlic, cooking until the onion is tender, about 2 minutes.
Very slowly add the yogurt mixture, stirring constantly, and bring it to a boil, cooking about 4-5 minutes.
Make sure you stir the yogurt or it will curdle.
Reduce wok heat if ingredients start to boil over.
Once liquid comes to a boil, reduce heat to medium, and simmer, stirring occasionally, until sauce is smooth and silky, and oil becomes visible, about 45-50 minutes.
Adjust thickness of sauce with the exta cup of water, if needed (cook further if it is too thin).
Sauce can be made up to 2 days ahead of time.
Stir in the fenugreek, then add spinach, and simmer about 5 minutes more.
Check consistency of sauce, and thin if necessary.
Heat remaining oil in a saucepan over medium-high heat and briefly heat dried peppers (about 30 seconds), shaking pan; stir in coriander and remove from heat (alternatively, for an oil-free method, dry roast peppers and coriander briefly in pan, but do not toast the paprika).
Add paprika and IMMEDIATELY pour mixture over the curry; do not delay or the paprika will burn.