Remove pin bones from salmon. Place salmon skin side down in a container lined with a large piece of plastic wrap. Mix together the Mexican seasoning, sugar, salt and tequila and rub into the flesh of the salmon fillets. Place the coriander on one fillet and sandwich with the other piece of salmon end on end so the pieces fit together neatly like top and tailing. Wrap the plastic wrap around very tightly and place in a plastic zip-lock bag, then put back into the container and place a heavy weight on top (I use an old paving brick wrapped in tinfoil; you could also wrap some heavy cardboard in tinfoil and weight it down with several heavy cans). Refrigerate to cure for 18 hours. Combine the mayonnaise and lemon zest and juice, and refrigerate until required. Remove the salmon from the refrigerator, discard the coriander, wipe off excess marinade and thinly slice the salmon. Place on a platter and garnish with lemon mayo and fresh watercress.