Thickening your curry sauce is easy. Depending on your personal preference, or what you have available in your pantry, you can thicken your curry sauce with very few steps.
Simmer your curry sauce for several minutes. Sometimes all your sauce needs to thicken is more time. Leave the lid off your saucepan or skillet to allow the moisture to evaporate, instead of dripping back into the sauce. This method is called "reducing" and is a good way to thicken a sauce without changing the flavour.
If that doesn't work, heat 15 gr (1 tbsp) butter or vegetable oil in a small saucepan. Do not burn. Whisk in 5 gr (1 tbsp) flour and take off the heat. You will end up with a pasty mixture. Take 1 cup liquid out of your curry sauce mix and whisk it into the paste. Blend this flour and sauce mixture into your original sauce and stir occasionally until the sauce thickens. It will take 5 to 10 minutes.
Another solution is to mix 8 gr (1 tbsp) cornstarch into 125 ml (1/2 cup) water or until the consistency is to your liking. Blend into your sauce and cook for 5 minutes.
Instead of flour or cornstarch, dice a potato and add it to the sauce. Cook until the potato begins to break down. The starch in the potato will act as a thickener without diluting the original flavour too much. You can add a well-drained, chopped tomato or 15 to 20 gr (3 to 4 tbsp) of tomato paste for a similar effect.
Arrowroot powder may be used in place of cornstarch, if corn allergies are an issue. You may also want to mix in another tablespoon of curry paste, but this may spice up the sauce more than you want. Dilute the added curry flavour by adding another tomato or potato.
One thing to remember about thickening any sauce is to have a patience before continuing to add thickening agents, or it will become too thick, and you will end up looking up how to thin the sauce.