Two or more veg in one space!

All plants need light, that is what they grow leaves for after all, feeding the starving millions is a bonus.
So whatever you grow together should not block the light for the others - for too long. Onions and to a lesser extent carrots are adversely affected if left smothered with weeds for any length so your lettuce would need to come out (or leaves picked) if too bushy and the onions would need to be far enough from the carrots that they were in good light till needed.
The main ones I plant together are sweetcorn (and now quinoa) and squash/pumpkins.