If you’ve ever made gravy, or any sauce or similar food to which a thickening agent is added, you know that it acts quite differently once you take it out of a hot, simmering pot and get it on a plate. Gravy, jams, and many sauces all thicken as they cool, and it can be hard to tell when they’re “done” while they’re bubbling away. Instead of relying on how they coat a spoon, dollop a little on a cold or room-temperature plate and see how it behaves. (If you’re happy with the spoon trick, great, but I always get stuck on whether or not the spoon is coated enough, and I just end up staring at spoons for way too long.)
When choosing the temperature of your plate, think of how you intend to store or serve whatever you’re cooking. Jam makers test their wares with a cold plate to see how they gel, which makes sense because jams are usually stored in a cold environment. A little bit of jam is placed on a plate that’s been in the freezer for five minutes, then the plate is held vertically. If the jam descends slowly, it’s done. (You can also put the jam on a plate and then put the plate in the freezer. If it sets after few minutes, you should be good.)
For sauces and gravies, you don’t need the plate to be so cold. Both are usually served on room-temperature (or slightly warm) plates, so it makes more sense to test them on those, rather than on something that has been chilled. Once you think your gravy or sauce is reaching its “end point,” take a spoonful and pour it on a room-temperature plate, let it sit for a few seconds and observe how it behaves. If it’s a little runnier than you would like, cook it longer, then repeat until it’s thickened to your liking.