Preheat the oven to 120°C / 235°F / Gas Mark ½). Line a baking tray with baking paper.
Place a piping bag and the star nozzle in the freezer for a few minutes so they are well chilled.
Bring the lemon juice to a bare simmer in a saucepan, then turn off the heat.
Whisk the eggs with the cornflour and agave syrup in a bowl until the mixture is smooth.
Pour into the saucepan and thicken over medium heat, stirring constantly with a wooden spoon.
Allow to cool in the refrigerator for 30 minutes.
Whisk the egg whites to firm peaks with half the sugar.
Add the remaining sugar and mix gently. Using a tablespoon, place small mounds of meringue mixture on the prepared tray, spaced far enough apart so they don’t stick together. Bake for 20 minutes at 120°C / 235°F / Gas Mark ½), then 1 hour at 100°C / 200°F / Gas Mark ½. Once the meringues have cooled, break them into pieces.
Work the butter and sugar together with your fingertips.
Add the flour and almond meal, then work together to make a crumbly mixture. Refrigerate for 10 minutes, then spread the mixture on a baking tray and bake at 200°C / 400°F / Gas Mark 6 for 15 minutes.
If the frozen yoghurt is in the freezer, take it out 10 minutes ahead of time.
Fill the cold piping bag (star nozzle attached) with yoghurt.
Pipe out the yoghurt soft -serve style and sprinkle with the broken meringue and the crumble.
Pipe small balls of lemon curd onto the frozen yoghurt with the other piping bag (plain nozzle attached).