The correct French spelling is

chef tournant

"The Relief cook (le Chef de Tournant)"
http://website.lineone.net/~eadam/classica.htm

However, I would leave it in French, since it is so typically French, the same way the word "chef" which is French is used in English. But you could use the words "relief cook" in parenthesis, so that people understand what this is.

Here are some examples:

"Beginning with
the position of chef tournant at the Boston Ritz-Carlton
during college, Rubin has spent nearly two decades
gathering experience in the kitchens of luxury resorts and
hotels in the United States and Russia."
http://www.nhb.org/foodserv/chef/rubin.html

"With the Associate in Applied Science degree, you can become an assistant kitchen manager, chef de partie, caterer, banquet
chef, chef tournant, sous chef, and eventually, executive chef. "
http://www.tc.cc.va.us/programs/ot/bt/culinary_arts.htm

"After school, he apprenticed at the Greenbrier Hotel in White Sulphur Springs, W.Va. He also worked as chef at the Ice House Cafe in Herndon, Va., and as chef tournant at La Chaumière in Los Angeles. "
http://dsc.discovery.com/dscdaytime/greatchefs/profiles/wong...

And there are dozens of other examples available on the Net.

Forgot to say that yes, this is the right spelling for word you are looking for. Here is the complete definition found on one of the sites mentioned previously:

"
The Relief cook (le Chef de Tournant)

The Chef Tournant usually relieves the chefs of the sauce, roast, fish, and vegetable parties on there day off. The first commis in the larder and pastry usually relieves his own chef. in some places a commis tournant will also be employed."
http://website.lineone.net/~eadam/classica.htm