My daughter is on a gluten free diet. If I wanted to use corn starch to replace flour as a thickener in stews, how much would I need?
As a general rule, you would use about half to two-thirds the amount of pure starch, depending on type, as flour to get the same thickening effect.
What I would suggest, is that for each tablespoon of all-purpose flour called for as a thickener, make a slurry with two teaspoons of corn starch and enough water to make a smooth mixture about the thickness of light cream. Add half of the slurry to the dish being prepared and cook until the mixture is almost boiling. Corn starch will thicken at a lower temperature than flour. Check the consistency and if necessary add more, reheating and stirring each time until you get the result you are looking for.
Also, remember that as the dish cools down during service, the sauce will become more gelatinous if you use corn starch, and that the sauce in any leftovers will become thinner if you freeze or reheat the dish.
Corn starch offers the closest result to flour in making stews, but you can also use arrowroot, tapioca flour or potato starch. These will all produce a clearer sauce, and potato or tapioca starch may result in a slightly gummier or more stringy consistency than flour or corn starch.