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:
Buy a fresh bright green firm bunch (usually five or six shoots) line them up and cut off the tops with one slice so that they are even. Don't be concerned that you are discarding too much (I do this step both for checking purposes and for kashrus since you don’t know where their knife has been).
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.
I will post again if you’re interested for my experience and procedures for strawberries, broccoli and cauliflower.
Remember, it’s much easier and more accurate for an individual (you) to check for bugs than for an institution or business (such as Bodek) to check large volumes of leafy vegetables. So why depend on Bodek when you are more reliable and don’t have to employ heteirim for your product.