In a large mixing bowl add flour, half salt, pepper, parsley, chili powder, paprika, baking soda & baking powder. Whist until combined.
Remove half of flour from mixing bow, and pour onto a plate. Add in panko breadcrumbs to the plate of flour, combine and set aside.
Cut the pieces of fish into bun-sized squares, pat dry and set aside. Add vegetable oil to pan and heat on medium.
When the oil is ready (approximately 350°-375°) add vinegar and your choice carbonated liquid into the bowl of flour. Mix slowly to not disturb the air bubbles. The batter should resemble a thin pancake batter.
Dip each piece of fish into the batter and shake off the excess. Dredge the fish into the panko flour mixture.
Slowly submerge the fish into the hot oil and fry flipping on both sides for about 7-9 minutes or until golden brown.
Once the fried filets are done, place them onto a wire rack lined with paper towels and a sheet pan. Sprinkle with the remainder of the salt.
Butter you brioche buns and toast both sides until golden.
Assemble your sanwhices: bottom bun, fried filet, cheese slice, slaw, tartar on top bun (add to bottom bun if you're feely saucy) and ENJOY!
(Optional) slice an onion and toss pieces in batter and panko mixture and fry.