In a food processor or blender, combine 3 cups of plain nonfat yogurt, 4 cups of water, 1/2 cup of flour, 2 tablespoons of ground coriander, 3/4 teaspoon of turmeric, and 1 teaspoon of salt.
Blend in 2 batches due to the volume and set aside.
Ensure the yogurt is thoroughly blended to prevent curdling during cooking.
Heat 2 tablespoons of vegetable oil in a large wok or saucepan over moderately high heat. Add 5 serrano peppers and cook, stirring, until they sizzle (about 30-45 seconds).
Be cautious of white spots on peppers and potential bursting.
Add 1/4 cup of minced onion, 1 tablespoon of minced ginger, and 1 teaspoon of minced garlic. Cook until the onion is tender (about 2 minutes).
Slowly add the yogurt mixture to the wok, stirring constantly. Bring it to a boil and cook for about 4-5 minutes.
Stir the yogurt continuously to prevent curdling.
Reduce heat if ingredients start to boil over.
Once boiling, reduce heat to medium and simmer, stirring occasionally, until the sauce is smooth and silky, and oil becomes visible (about 45-50 minutes).
Adjust the thickness of the sauce with an additional cup of water if needed. Cook further if it is too thin.
The sauce can be made up to 2 days ahead of time.
Stir in 1 teaspoon of dried fenugreek leaves, then add 5 cups of chopped spinach and simmer for about 5 more minutes.
Check the consistency of the sauce and thin if necessary.
In a separate saucepan, heat the remaining 2 tablespoons of vegetable oil over medium-high heat. Briefly heat 5-7 hot red chili peppers (about 30 seconds), shaking the pan. Stir in 2 teaspoons of ground coriander and remove from heat.
Alternatively, for an oil-free method, dry roast the peppers and coriander briefly in the pan, but do not toast the 1/2 teaspoon of paprika.
Add the paprika and immediately pour the mixture over the curry to prevent the paprika from burning.
Serve the Spinach Curry with Yogurt Sauce immediately with steamed basmati rice.
