Start by preparing the sauce. Add all the sauce ingredients to a small bowl and mix well. Cover and store in the fridge.
Devein and rinse the prawns then pat dry with paper towels. Add them to a bowl then add the salt, black pepper, paprika, cayenne pepper, garlic powder, onion powder, ground ginger, thyme and oregano. Mix well so the seasonings coat all the prawns then set the bowl aside.
Gather 3 bowls and add the plain flour to one bowl. To another, add the egg and sriracha then whisk them together. To the last bowl, add the panko breadcrumbs. Take a prawn, dip it in flour then dip it in the egg mixture. Drain off any excess egg then dip the prawn into the panko breadcrumbs and turn the prawn a few times to fully coat. Place the prawn on a plate or baking tray and repeat the process with the rest of the prawns. Set the breaded prawns aside to rest for 15 minutes.
Place a large frying pan or skillet onto a medium heat and add the oil. Allow the oil to heat then add the prawns. Cook for 5 minutes turning occasionally until the prawns are golden brown. If the prawns begin to brown too quickly then the heat is too high and you should reduce the heat. If 5 minutes past and the prawns are not golden then increase the heat slightly. Remove the prawns and add them to a plate lined with kitchen towel to soak up any excess oil.
Finely chop the lettuce and slice the tomato. Cut the brioche rolls longways and spread the sauce onto both sides of each roll. Add in the lettuce, tomatoes and prawns in any order you want. If you prefer, you can assemble the sandwich then add the sauce on top instead.