Thin or runny curry is often the result of using unthawed frozen vegetables or adding too much cooking liquid. Attempts to tone down a too-spicy taste with coconut milk can remove some of its heat, but also cause the sauce to become thin. While thin curry is undesirable, it is not a reason to panic. There is more than one way to thicken curry and in the process restore its thick, saucy texture.
Turn your stove burner to medium high, remove the cover from the saucepan and let the liquid simmer just below boiling point for about 5 minutes without stirring. Simmering curry without additional thickening ingredients allows excess liquid to evaporate into the atmosphere and is a good choice for sauce that is only slightly thin. Watch the sauce closely during this time and, if necessary, adjust the heat to ensure the sauce does not boil.
Stir 1 tbsp. of arrowroot into the saucepan for each cup of liquid. Arrowroot is a flavor-neutral thickener that works much like cornstarch. Turn your stove burner to medium-high and stir constantly until the sauce starts to thicken. This method is a good choice for sauce that is thin but not runny.
Heat a thickening paste consisting of 2 tbsp. of flour and 2 tbsp. of melted, unsalted butter for each cup of curry sauce in a separate saucepan until the butter just starts to turn brown. Pour in 1 cup of the thin curry sauce and mix well. Add this mixture to the curry sauce, bring the liquid to a boil, then reduce the burner heat and simmer for 4 to 5 minutes. Use this method if your sauce is thin to the point of being runny.