Cook the Base Ingredients: It is important to cook the aromatics in hot oil or ghee so that their moisture cooks out and can then form a thick paste with the spices. Cook until the oil separates, which is a sign the base is ready for the liquid to be added.
Simmer/ Reduce: Continue cooking the curry, uncovered, over a low simmer until thickened. Stir occasionally to prevent burning. The longer you simmer the curry, the thicker it will become. This is my preferred method, but a slow one. Use one of the methods below if short on time.
Add Tomato Paste: Whisk in 2 tablespoons tomato paste and simmer for an additional 4-5 minutes. It's a great option for tomato-based curry sauces.
Mash or Puree Vegetables: Mash or puree 1-2 boiled potatoes, squash, or carrots. Add to the curry and simmer for 5-10 minutes more, or until thickened.
Puree a Portion: Take a ladle full of curry with plenty of chunky vegetables (refrain from adding the meat) and blend it. Add the blended portion back to the curry and simmer for 2-3 minutes, or until thickened.
Greek Yogurt or Heavy Whipping Cream: Remove the curry from the heat (or set over the lowest heat setting) and stir in 2-3 tablespoons of full-fat Greek yogurt or heavy whipping cream. Whisk until smooth and combined. It's important to add the yogurt/ cream over a low heat setting, or the dairy will curdle.
Coconut Cream or Coconut Milk: Whisk in 2-4 tablespoons of full-fat blended coconut milk or cream. Simmer until slightly thickened. Coconut cream is best used for Thai or Southeast Asian curries.
Grated/ Shredded Coconut: Add 2-3 tablespoons grated/ shredded/ desiccated unsweetened coconut to the curry sauce. Simmer for 2-3 minutes, or until thickened. Fresh coconut is best for Thai or South Indian curries.
Cashew Paste / Ground Nuts: Make a quick nut paste by soaking a handful of cashews or blanched almonds in hot water for a few minutes. Blend until smooth, then whisk into the curry. If almond flour is all you have on hand, stir in 2-3 tablespoons and simmer until thickened. Poppy seeds and melon seeds are also common thickening agents, but these are usually harder to find in stores.
Make a Roux: Melt 1 tablespoon of butter in a saucepan over medium heat. Add 1 tablespoon of all-purpose flour. Cook on low heat until the mixture forms a paste, then continue to cook until it has a nutty aroma. Add the roux into your curry with constant stirring. It should thicken right away.
Cornstarch/ Arrowroot Slurry: Combine 1 tablespoon cornstarch or arrowroot flour with 2-3 tablespoons water or curry liquid, and whisk until smooth. Add to the curry sauce and simmer for 2-3 minutes to rid of the raw flour flavor. Note: It's essential to mix the starch with liquid prior to adding it to the curry. The flour will create lumps if added directly into the saucepan.
Pureed Lentils: Add 2-3 tablespoons of pureed cooked lentils to the curry. Simmer for 2-3 minutes, or until thickened. The lentils will alter the flavor slightly, but boost the nutritional value, too.
Peanut Butter (Or Nut Butter): Whisk in 1-2 tablespoons unsalted natural peanut butter (or cashew butter or almond butter) when the curry is done cooking. The slightly roasted peanut flavor works best in Asian curries, especially Thai massaman, or panang.
Notes
How to prevent a watery curry, to begin with:
Don't add too much liquid or broth during cooking.
Simmer the sauce for 2-5 minutes after cooking.
Cook on medium-high to high heat to cook off excess liquid.
Cook the curry without the lid after the ingredients have cooked through.
Account for excess water or juices from the ingredients.