Lemon mayo: Whisk egg yolk, vinegar & salt together. Slowly pour in oil, whisking continuously. Beat in pulp of preserved lemon.
Whitebait fritters: Beat egg white to foam with salt. Add chopped parsley & whitebait & fold to combine. Heat a frying pan to medium heat with butter. Cook 12 tablespoons of whitebait batter to an even golden colour on both sides. Transfer cooked fritters to a warm plate & cover to hold.
Turn frying pan to low & place blinis in to warm through. When warm put a dollop of mayo on each blini, a table spoon of watercress on half of the blinis, top with a fritter & then put remaining blini on top to make a sandwich. Use skewers to hold in place & serve on warm platters.