Most firm vegetables can be turned – potatoes, carrots, swedes, etc. This French technique serves a dual purpose – the vegetables will cook evenly and the presentation is elegant. Actually, it’s a bit of a faff so I would only bother with this if you’re planning on emulating fine dining in your home. That said, most catering students have to go through the hell of learning tournage de légumes so it’s up to you how serious you want to get about these things.
To turn vegetables you’ll need a sharp paring knife or a turning knife (I don’t have one of the latter) and, in the beginning, a lot of patience. You can use the off-cuts for soup or casseroles.
Turned potatoes play an important part in French cuisine and have been codified by size, each bearing its own name. L’anglaise describes potatoes which are 5 cm (2 inches) long and 2.5 cm (1 inch) thick. Cocotte refers to those which are a bit shorter and more olive-shaped. Château refers to those which are a bit thicker and longer than pommes de terre à l’anglaise. There are other sizes but if you need to know them you probably already do and Not Delia doesn’t have the time or inclination to chunter on about it. By the way, each barrel-shaped vegetable should have seven sides.
This is a kinda boring subject but James Martin makes it quite fun. You can watch him turning vegetables on the video below.