Don't be afraid of the alternatives
Cornstarch is one of those remarkably adaptable pantry items that can do a lot of things very well, including thicken sauces, bind baked goods like pies fillings and custards, and help crisp up frying meat or vegetables. But sometimes, you need a cornstarch substitute—maybe because you're cooking for someone who's allergic to corn, or maybe because you just ran out of cornstarch mid-recipe and don't have time to run out to the market. And though no one ingredient has exactly the same versatility as cornstarch, there are a few good substitutes for cornstarch out there, some of which you probably already have in your kitchen cabinets.
The most obvious substitute for cornstarch—and the cornstarch substitute you're most likely to have in your kitchen already—is flour. For every one tablespoon of cornstarch, you should use three tablespoons of flour, according to Epicurious. If you're using flour as a thickening agent in a sauce, you do have to cook it a little longer than you would've if you had used cornstarch; that's in order to get rid of the flavor of flour. Flour also won't give the sauce the same gloss as cornstarch does, but it'll have a really similar consistency, which is really what you're looking for at the end of the day.
If you're in need of a gluten-free cornstarch substitute, you have a couple of options. The first is tapioca flour, also known as tapioca starch. According to Bob's Red Mill, which manufactures and sells tapioca flour, it's made from grinding cassava root. It also gives a super high-gloss to your sauces, which makes it great for pie fillings.
The other gluten-free substitute for cornstarch that's commonly used is arrowroot flour, though be careful when adding this thickener to dairy-based sauces. According to the Cook's Thesaurus, arrowroot flour can give those milky, dairy-based sauces a slimy consistency rather than a thicker one. But arrowroot flour is a great thickener for acidic sauces, like those with a tomato base.
So sure, cornstarch might still be your ride-or-die thickener—but don't be afraid of these cornstarch substitutes, either, because in some circumstances, these alternatives might actually be better than the original.