Whisk the egg whites with the lemon juice and a third of the caster sugar until stiff. Gradually add the remaining two-thirds of the caster sugar. Put in two piping bags: one with an 11 mm nozzle and the other with a Saint-Honoré nozzle.
Arrange the meringue on a plate covered with Silpat®: first pipe a 24 cm disc with a piping bag fitted with the 11 mm nozzle, then petals with the Saint-Honoré nozzle round the edge. Also pipe some meringue petals for decoration separately.
Bake for 2 hours at 90°C and then for 2 hours at 70°C. Remove from the oven and place on a wire rack. Allow to cool completely.