Pre-render layer

There's Composition > Pre-render..., but you might be thinking of setting a Comp Proxy.

1. Select the Layer that has a nested Comp as the source.
2. Right-click and choose Reveal > Reveal Layer Source in Project.
3. From the Project panel, add the revealed Comp to the Render Queue.
4. Make sure that the Output Module template is set to High Quality (if there is no transparent area in the nested Comp) or High Quality with Alpha (if there is trasnparent area in the nested Comp).
5. Click the Output Module name to open the Output Module Settings dialog box.
6. In the Output Module Settings dialog box, change the Post-Render Action pop-up menu from "None" to "Set Proxy".
7. Save and render.
   When completed, the Proxy switch will show to left of the name of the nested Comp in the Project panel and the High Quality movie is being used instead of the Comp as the Layer source.

The default Proxy Use setting in the Best Settings Render Settings template is "Use No Proxies".  So to take advantage of using the Comp Proxy when rendering the containing Comp, be sure to enabled "Use Comp Proxies only" for the Proxy Use pop-up menu in the Render Settings.  

Do you not see the command Composition > Pre-render when you select your comp in the Project panel?