Tuna, chicken, ham, whatever, without all of it squishing out??
Good serrated knife, but I still end up trying to jam the filling back in. Said sandwiches are on toast.
First I need to know what a salad sandwich is.
Good sharp knife… don’t apply too much pressure, let the knife do the work. Never been a problem.
Use a bread knife and don’t squish. Or, you could just cut the bread before assembly.
Start the sawing motion before the knife touches the bread, so it’s doing the work, and you’re not mashing the knife into the sandwich. I think you’re mashing it without realizing it. Is the serrated knife sharp for sure? I’ve come across many a dull one because its owner doesn’t think it needs to be sharpened because it’s serrated.
Per the OP…
Tuna (Salad)
Chicken (Salad)
Ham (Salad)
Whatever (Salad)… Egg, Turkey, Lobster, Crab, Salmon, etc.
Try wrapping the sandwich in something like butcher paper (paper towel, maybe?) before slicing.
AH – thanks! I go with open-faced on a bagel or toast and avoid the issue. 
Lay down one slice of toast. Add the toppings. Cut the second slice into two and put on the top, then slice through the rest of the sandwich with the cut lined up with the pre-cut slice. It sounds goofy, but it works.
Diagonally, of course.
Genius!
YES!!!
Even easier… start with your SHARP serrated knife (Hint, if you inherited your parents knife that was a wedding present it probably isn’t sharp anymore). Assuming you are right handed, hold the handle lightly in your right hand and lay diagonally across the sandwich. Take your left hand and gently place your palm (heel) on one corner and the finger tips on the opposite corner (arching over the knife). Start sawing gently across the bread applying minimal pressure. Apply just enough pressure with your left hand to counter any bowing of the toast. A few bits may pull out under the corners of the knife… simply put them back when done.
Sounds a lot more complicated than it is… takes a few seconds in real life.
Straight-edged knife (bigger than the bread, diagonally). Make sure the salad isn’t all the way to the edge of the bread (maybe 1/4" of space). Place knife evenly, diagonally, hand on top of the blade, and press down all at once with a quick, firm motion, then wiggle it slightly to separate the pieces.
Crimps the inside edge of the sammich, pressure squishes the salad out to the outer edges of the bread without spilling over.
Serrated (bread) knife, never had a problem. Maybe your salad is too squishy?
I will admit that one serrated knife I have works better than another, so maybe the style of serrated knife edge is important.
Use the tip of the knife: cut down inside the crust and scalpel-cut to the inside far corner (this method is mostly for untoasted bread, for lunch-box sandwiches, so they keep their shape for five hours or so).
This is, as was pointed out by others, brilliant!
You are truly a Goddess.
I can’t wait to show this technique off this weekend when I have guests.
Thanks!
This is what I do - start with the tip of the knife and make a few “stabby” cuts, then cut normally the rest of the way through.
If you have the room in your kitchen, a laser cutting table might work.
An Ulu works well, as does a pizza cutter. If the bread isn’t toasted, I start in the centre and work my way out to the crust.