Just how thick do you want your curry? If you’ll be serving it over rice or another absorbent base, the curry should be thick enough not to run off the rice onto the plate, but not so thick that it forms an unappetizing glutinous mass atop the rice. The platform of rice should be able to absorb a good part of the curry sauce, but not to the point of getting soggy. Whether you’re starting out with a basic curry stock or you’re making the curry dish from scratch, the important thing is to get its consistency just how you want it.
Making Your Own Curry Stock
Many curry-loving home cooks make up a big batch of a basic stock, typically composed of a blend of seasonings mixed into flavored broth or plain water. For example, they might use a puree of tomato, onion, garlic, hot chili paste, turmeric, coriander and curry powder. This curry stock can be frozen for future use and adjusted every time you make a different recipe.
Pour the stock into ice cube trays and store the frozen cubes in freezer bags. You can take them out as you need them. The thawed cubes can be used as a base for a variety of curries or for a rich curry sauce “gravy” to be served over meat or vegetables. This gravy, though not an entree in its own right, can be thickened using some of the same techniques as main-dish curries.
Do not attempt to freeze your curry stock if you’ve added dairy products or coconut milk, which do not freeze well.
As the Pot Thickens...
Whatever you've used as the base for your curry, you will eventually wind up with a nice pot of aromatic goodness filled with seasonings, vegetables and some form of protein. Simply simmering the curry will allow it to reduce, thereby thickening it. Sometimes, that’s all it takes.
If you’re making a creamy curry (korma, for example), you’re most likely adding in either coconut milk or some form of dairy product: yogurt, sour cream, heavy cream, etc. Which one you choose depends, of course, on what flavor profile you’re aiming for. It's quite possible to thicken curry with cream or other dairy products, but be careful to keep the sauce on a low simmer. Boiling the sauce with these ingredients – including coconut milk – can cause it to break up.
Thickeners That Don't Add Flavor
There are thickeners you can add to your curry without disturbing the flavor you’ve already achieved, and they’re probably already on your shelf. For instance, dissolve a teaspoon or so of cornstarch in enough water to make a slurry. After making sure the sauce is not bubbling up, carefully add in the slurry. It’s a good idea to remove the pot from the heat first, since in a very hot pot, the cornstarch mixture can coagulate into lumps.
You can use the same method with flour. You’ll need more flour than you would cornstarch. Again, make sure the flour is completely dissolved in the slurry and add it slowly to your sauce. Alternatively, you can ladle out some of the simmering liquid, let it cool for a few minutes, add the cornstarch or flour, then return to the pot. This will thicken the sauce without diluting the flavor.
Thickening With Vegetables, Oatmeal and Nuts
If there are vegetables in your curry, puree some of them in a blender; then add back to the pot. This works especially well with potatoes and other vegetables with a large amount of starch (which acts as a natural thickener).
Add grated raw potato or quick-cooking plain oatmeal to the pot 30–40 minutes before serving. Either of these will add flavor as well as thickness. They won't necessarily work with every kind of curry, so keep in mind the kind of flavor profile you're creating.
Pulverize cashews in a blender or grinder; form a paste with water or some of the pot liquid and add back in. This will add a nice nutty nuance to your curry and is especially complementary to creamy curries.
Will Sauce Thicken in a Slow Cooker?
Curry in a slow cooker won’t reduce as much, because it’s a closed container by definition, and steam doesn’t escape but rather forms as condensation inside the lid. Still, you can try using the cornstarch/flour thickening method in a slow cooker. Add the slurry about 20–30 minutes before serving.
You can also take the lid off the cooker or set it ajar during the last 30 minutes of cooking, thus forming a vent for steam to escape. No matter what method you use with your slow cooker, do check it a few times to make sure it hasn’t over-thickened.