In Final Cut Pro under the View menu for External Video, there is an option for Single Frame. The keyboard shortcut listed for this is F15. My keyboard does not have a F15 key.

The Single Frame function can be useful for composite or other operations that can take a long time to render. Switching External Video to Single Frame will allow you to output the video to the program monitor when you depress F15 or the Shift and F12 keys.

Note: On the USB Apple Keyboards that ship with Power Macintosh G3 and G4 computers, there is no F15 key. Depressing the Shift and F12 keys will give you the same results.