First make the mayonnaise: in a medium bowl, mix the soured cream, mayonnaise, milk and a pinch each of salt and pepper, or to taste, then set aside.
To make the batter, put the self-raising flour in a wide bowl, add the salt, oregano, mustard and beer, and whisk until there are no lumps.
Put the plain flour on a plate, season and mix. Pat dry the prawns with a kitchen towel, dust each prawn in the seasoned flour, shake off the excess, then drop into the batter.
Put the oil in a medium frying pan on a medium heat. Once it’s hot, fry the prawns in batches for a minute or so on each side, until golden brown, then transfer to a plate lined with kitchen paper to drain.
Heat the tortillas in a hot, dry frying pan, then transfer to a warmed plate and cover with a tea towel to keep them warm.
To serve, take one warm tortilla, spread it with some mayo and lay three battered prawns on top. Add some shredded lettuce, pico de gallo, pickled onions, a squeeze of lime and, if you want some heat, spicy salsa, and tuck in.