Preheat the oven to 200C/Gas Mark 6. Grease and line a 30x20cm (12x8in) Swiss roll tin. Put the eggs into a large bowl standing over a saucepan of hand-hot water. Whisk for 2 minutes. (If you are using an electric mixer, no heat is required during whisking.)
Add the sugar and whisk for a further 8-10 minutes, or until the mixture is very light in colour, thick in texture – consistency of softly whipped cream – and at least double its original volume.
Remove the bowl from the saucepan. Continue whisking until the egg mixture is cool. Gently fold in the flour with a large metal spoon, then transfer to the prepared tin.
Bake for 10-12 minutes, or until well risen and firm. 5 Turn out onto a sheet of baking paper, then carefully peel off the paper. Cut away the crisp edges with a sharp knife.
Spread quickly with warm jam or curd, then roll up tightly and hold in position for 1 minute. Cool on a wire cooling rack.