Several options, depending on the type of curry and the ingredients already present.
Japanese Style Curries Using a commercial, packaged Japanese-style roux: Add another brick or two from the package. This type dissolves nicely generally with minimal clumping. Using a homemade
, Japanese style roux:
You can prepare additional roux by melting fat (butter, beef fat, etc) in a saucier, foaming it if needed (as with butter), and whisking in flour (we usually use a 1:1 ratio).
Allow the flour to brown to the desired level, keeping in mind that the thickening power decreases the darker it gets.
If you prefer not to use flour, potato starch/katakuriko is a reasonable alternative.
When suitably dark, take some of the liquid from your existing curry and mix in to the roux, whisking vigorously to minimize the risk of clumping.
Add more of the existing curry liquid if you think there's a risk of clumping, then reintegrate it back into the main pot.
(if you were starting this roux from scratch you'd also integrate fat-cooked spices into the roux, but your problem does not appear to be flavor).
Avoiding fussiness:
Mix cornstarch or potato starch (katakuriko) in cold water.
Once reasonably dissolved, add it to the curry pot.
This type of thickener can create a slightly odd texture in a Japanese curry, as the browned roux creates a more velvety texture, but that doesn't mean I haven't done it before in a pinch.
The liquid will thicken as it boils.
Indian Style Curries
For more Indian-style curries, there are other options.
If your curry contains lentils, chickpeas or potatoes: Smash some of them in a small bowl, or just against the edge of the pan.
Allow the pot to boil a bit.
If you have almonds, cashews or similar nuts available: Grind a small quantity of nuts in a coffee/spice mill, and incorporate them into the curry.
These need to boil more gently, and I usually do this sauce more intentionally by making them the focal point (dum ki ghom, for example, which has mushrooms and tomato puree in a nut sauce) but can rescue an existing curry too.
cooking.stackexchange.com/users/2909/JasonTrue
In this case, probably adding more curry mix is the answer.
You may also need to use a little cornstarch to thicken.
It's worth bearing in mind that if you're using a recipe not designed for slow cookers, you should usually halve the amount of liquid it states.
cooking.stackexchange.com/users/4194/ElendilTheTall
You can always use cashew/almonds/freshly grounded coconut paste to thicken it up.
In most of the indian cooking that what used.
This will added up richness to your recipe.
Yes, cornstarch is another good option too.
cooking.stackexchange.com/users/23066/pushya
Rather than adding thickener why not reduce your sauce?
Separate the meat and vegetables, put the sauce in a wide pan and then cook it down.
It will thicken it up, and concentrate the flavors.
cooking.stackexchange.com/users/19707/GdD
Slow cookers trap moisture so frequently result in"runny" results.
Easiest, non interventionist solution: take the lid off for the last (or an additional) hour - more or less depending on results - on high.
cooking.stackexchange.com/users/23695/user23695
yes dissolve cornstarch with water then mix it.
if the curry/spices flavour start to decrease, add more curry paste/curryroux.
but i wonder how the the curry restaurant keep their curry thick as hell for their customer ? :/
cooking.stackexchange.com/users/44867/sisi
I made a lamb based Japanese curry in a slow cooker and unlike my previous efforts (using many of the same ingredients, though in different proportions), it turned out runny and underwhelming.  What techniques exist to "beef up" a curry after the fact? Would putting it back in the slow cooker with more veggies for another half-day help? Would adding another portion of curry mix help?
How do I thicken my Japanese curry after the fact?
cooking.stackexchange.com
<curry><thickening><japanese-cuisine>
cooking.stackexchange.com/questions/41826
cooking.stackexchange.com/users/5923/Brian Ballsun-Stanton
If using a slow cooker with a normal recipe, you can usually use half of the liquid it states, but you can thicken runny curry with a cornstarch slurry, more curry mix or cashew/almonds/freshly grounded coconut paste. Cooking in a slow cooker tends to result in thin sauces as the device traps moisture. To solve the problem, a thickener can be added, such as cornstarch or nuts, or the sauce can be reduced.
Cornstarch, curry mix and nuts can be used a thickeners. Slow cookers trap moisture and result in thin sauces. Reduce the sauce rather than add a thickener.