Cornstarch and agar -- the first conjures images of thickened sauces and soups, wheres the latter sounds more like a lab chemical than a common cooking ingredient. Both are carbohydrate-based food products used to thicken liquids. Cornstarch is used primarily for sauces and custards, or applications that result in soft gels. Agar, a gelling agent refined from seaweed, is used more often to create solid gels like those made from gelatin. However, both thickeners are plant-based and vegetarian friendly, and you can easily substitute cornstarch for agar in any liquid you want to thicken.
Measure 2 tablespoons of cornstarch for each 1 tablespoon of agar flakes called for in the recipe. If you're not following a recipe, measure 1 tablespoon of cornstarch for every cup of liquid you want to thicken.
Prepare the recipe and heat the liquid you want to thicken to a simmer. Whisk the cornstarch into an equal amount of cold water or stock.
Slowly add the cornstarch slurry, using a whisk or spoon to incorporate. The liquid should thicken almost immediately.
Add more cornstarch slurry if necessary to create the desired consistency. The liquid will thicken as it cools, so it may not be necessary.