If you are eating burgers, the iconic American dish, don’t mess with them.

You can get fiber in a dozen other different and appetizing ways.

I often sprinkle a couple of tablespoons of Flax seed on mine. I can’t see how it would hurt.

I put walnuts on my Greek yogurt and walnuts on raisin bread toast with cream cheese. I find them a good way to get additional fiber and protein.

Wheat germ, crushed peanuts or sunflower seeds I had them all on burgers.

I used to put wheat germ, and a generous amount at that, when I was young on my ice cream. Added a layer off texture that I liked. I had no idea what it was, but my Mom had done it so I did it too. i don’t remember it tasting like much, so I would assume a small amount on a hamburger wouldn’t affect the taste.

I doubt you would notice it. A little sprinkle won’t hurt the flavour but will add to your fibre intake. I add psyllium husk to stews and things and you would never know it is there.

I think I would mix it into the meat before making the burger. And maybe some wheat or oat bran.
@Bellatrix doesn’t the psyllium make things, well, goopy?