Prepare a 6 x 2 1/2 in (150 x 65 mm) deep souffle dish. Cuta5in (130mm) wide, double band of greaseproof paper to wrap around the dish snugly, but not too tightly, with an overlap. Secure with sticky tape. Stand the dish on a flat plate.

Finely grate the rind of the lemons and strain the juice. Separate the eggs, reserving 5 egg whites and 3 of the egg yolks. In a large bowl, whisk the 3 egg yolks with the caster sugar and lemon rind until very thick and pale. Gradually whisk in the lemon juice.

Put 3 tablespoons of cold water in a small saucepan, sprinkle the gelling agent evenly over the surface, and leave to stand until it swells and turns opaque.

Heat gently until the gelatine or agar agar dissolves completely and becomes quite hot, then remove from the heat and allow to cool a little.

Whisk the 5 egg whites until stiff, but not dry. Whisk the gelling agent into the lemon mixture, then carefully fold in the egg whites. Pour into the prepared souffle dish and level the top. Refrigerate for at least 2 hours, until firmly set.

Before serving, remove the sticky tape from the paper, then carefully peel the paper from the side of the souffle, pulling it back on itself against the back of a palette knife. If you wish, decorate the top with the Greek yoghurt or cream and pistachio nuts.