They have an identical form. But looking at them from different view, 90 degree in difference, they can be seen like upper lip and lower lip from the side, and form a month shape. The idea is to have one plate holding food and the other acts like a lid for food storage.
A little clueless about what line I should draw to create this shape. Any suggestions?
Many thanks!
use sweep2 with a circle which you cut in half as shown on the picture then use the left point then the curve then the right point and sweep.
for the inner part of the plate you can use revolve. distance those two surfaces from each other regarding your thickness and use blend surface.
Thank you Richard 
I finally settled down with patch and this is probably the best I can get for now.
looks good, you can also do it with sweep 2 and use several cross sections i in case you need more control. my example was of course just very crude.
