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Surprised we don't see more TV chefs (in Britain) espousing the use of thermometers, especially when cooking roasts and steaks.Takes the guesswork out of the cooking and it's something any home cook can do.
Most used is my bean to cup coffee machine data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7 but wouldn't call that a gadget....
Weber meat thermometer is also used a lot, but again that has become such a staple that I find it hard to call it a gadget especially as every kitchen really should have one.
Nope my nomination for a proper gadget is the apple slicer, super cheap from Wilko but oh so handy data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7
Wilko Apple Corer