I can just relate from my own experience. Without exaggeration, I have washed and checked thousands of heads of lettuce and scallions and other leafy vegetables over the years. I want to repeat – start with very fresh vegetables, it makes checking a lot easier and don’t be afraid to cut away and throw away. When you have leafy vegetables that are not totally fresh, then it can be hard to quickly determine leaf “rot” from what may have been a living bug.
With scallions:
Line up the scallion bottoms and do the same to the root, slicing them off slightly above where the root ends, with one motion.
Now slice of the greens just above and below where the scallion greens join together so that the joint is totally removed, and discard. The joint may be a hiding place that’s hard to check. Discard any leaves that are not totally green or fresh.
From the white lower part bulbs, peal off and discard the outer layer of scallion.
Take the upper bunch of scallion leaves which are now hollow tubes, and while holding loosely together in your fist, agitate briefly in slightly salty water. This procedure will clean your scallions and repeat usually once or twice until the water is totally clear (examine the water). Take leaves out of water and visually scan. You’re done.
Now throw the scallion bulbs into salty water and agitate in the water and repeat. You know have clean and kosher scallions.
Again, once you get the hang of the procedure, it goes fast.