Shred the cucumber, add to a colander and sprinkle it with salt, massage it for a few seconds, put a bowl under the colander and let it sit while you prepare the other ingredients.
Make the burgers: Shred the oyster mushrooms by hand then add them to a food processor with the S blade attached. Add all the ingredients for the fish-less burger and blend it by pulsing it 4 to 6 times. Do not overdo it, we need a flake-like consistency and not a paste. Now, shape the patties, put them on a plate and put them in the freezer for at least 20 minutes.
Now, finish the mayonnaise: In a jar, add the plant based milk, then add the oil, little by little while blending it. Stop adding oil when you notice that the mayo has become creamy. Then add the lemon or lime juice, salt, pepper and the apple cider vinegar. Now, squeeze the cucumber to remove all the remainder water, add it to the mayo along with the dill, capers, hot sauce and pepper. Put it in the fridge until you assemble the burgers.
Heat some vegetable oil in medium heat. Remove the fish-less patties from the freezer, bread them by passing them in the flour-water batter then in the panko bread. Fry the patties until golden brown, remove from the oil and place them in a cooling rack or paper towel.
Assemble your burger by adding a tiny bit of the cucumber mayo to the bottom bun so the lettuce sticks to the surface, then add the Fishes patty, the remainder of the mayo, cover with the top bun and enjoy it!