“Stir until thick” (or “stir until thickened”) might be one of the most difficult expressions in cooking to interpret when you’re just starting out. How can you tell when your sauce/custard/pudding is thick? How do you know it is thick enough? And what does “coat the back of a spoon” mean?
The reason that these questions are so confusing is that there is no standard definition of “thick” to go from. Let’s say that you’re trying to make a pudding and the recipe directs you to cook until thickened. The easiest way to achieve a thickened consistency is by cooking the pudding and watching over it. The milk and other ingredients will eventually come together as the mixture cooks. At some point, whatever is being used to thicken the pudding – eggs, flour, cornstarch, tapioca, etc. – will have added as much thickening power as it can and the mixture will continue cooking, but will no longer change in consistency. This is what “thick” is. It varies from recipe to recipe, and the product will range in consistency from somewhat syrupy to mayonnaise-like. The more you cook and the more you become familiar with the ingredients you’re using, the easier it will be to recognize when a mixture is “thick.”
Alternatively, you can cook a mixture until it “coats the back of a spoon” and use that as in indication that your dish is cooked. This direction does not mean dunking a spoon into a pot and seeing if anything sticks (every liquid will “stick” to some degree). It means that you should dunk a spoon into your pot, pull it out and run your finger down the back of the spoon. If the mixture does not run and the little path you have drawn holds it’s shape – as illustrated above with a photo of the pudding from my Southern Banana Pudding Tart – you have successfully “coated the back of a spoon” and thickened your pudding/custard/etc.