Onions - I do radial cuts vertically and I cut horizontally for a nice dice. I think this is the fastest way. Although, my chef will dice the onion by making batons and then dicing it like that which i think is ridiculous but whatever
Bell peppers - I roll them with out cutting the top or bottom off. The way I roll cut a bell pepper allows me to utilize as much of the pepper as possible and the only part left is the bottom which I cut after rolling it. If you do this right you can essentially skip the step of cleaning the ribs off of each panel of pepper.
Herbs - I bunch them up into a little bundle, as tight as possible, then I rock chop.
Fish Skin - I usually use whatever knife I have out whether it be my filet or my chefs/gyuto. I use a combo of sawing and pulling the skin while keeping the knife in one spot.
Tomatoes - For concasse, I cut the tomatoes in half and squeeze the seeds out then dice it up.