Hi, PamPam. A natural way to say that, is "I cook in a restaurant.
Obviously, it´s said in a general way.
If the restaurant is a big one, maybe you need or want to specify which position you work there.http://restaurants.about.com/od/kitchen ... g_Jobs.htm
The above it´s almost self-explanatory, but someone could play with your statement, since cook can be a "verb" and a "noun".
I mean, if you "cook in a restaurant.
" (cozinha em um restaurante), automatically one will think you are a cook
(um cozinheiro) there.
So, one will know from the context if you are reffering to one or the other sense/meaning of "cook".
If you had to give a short answer (with no time to contexts), you could say "I am a cook in a restaurant
Again, certainly nobody is going to be such a grammar nitpicker, but then, just in case...
Ah, I was going to forget, there is not such a thing as "a cooker"