These days I make my own sandwich fillers, and I have done for a long time.
Experimenting with leftovers and a dash of mayonnaise is pretty much guaranteed to make a sandwich filler that is tasty, quick and much better for you than a jar of shelf stable long life 35% vegetable mush.
I tend to use a couple of tablespoons of my choppy salads when I have some in the fridge, it's easy, quick and very tasty. Chopping the already chopped up salad veggies a little bit smaller and adding just salt, pepper and a squeeze of mayonnaise ... either bought or homemade ... and this time I added some dairy free cheese that I had ready grated in the fridge. Within minutes I had a fresh tasting and relatively healthy lunch.
If I had to eat some of the jarred stuff I think I would load the sandwich up with lots of crunchy lettuce and cucumber and try not to think of wallpaper paste while I ate my lunch.
What is your absolute favourite sandwich filler?