Lightly grease a 33 x 23 cm (13 x 9 inch) Swiss roll tin. Line with a single sheet of greaseproof paper to come about 1 cm (½ inch) above the sides of the tin. Lightly grease the paper.
Whisk the eggs and sugar in a large bowl over a saucepan of hot water until pale and thick. Sieve the flour and fold into the egg mixture with the measured hot water. Pour the batter into the prepared tin and bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 8 minutes until golden and set.
Meanwhile, place a sheet of greaseproof paper 2.5 cm (1 inch) larger all round than the Swiss roll tin on a clean damp tea towel. Once cooked, turn out the Swiss roll immediately face down on to this second sheet of greaseproof paper. Carefully peel off the lining paper. Roll the sponge up tightly with the new greaseproof paper inside. Wrap the tea towel around the outside and place on a wire tray until cool, then unroll carefully.
Add half the strawberries to the fromage frais or yogurt and spread over the sponge. Roll the sponge up again and trim the ends. Dust with icing sugar and decorate with a few strawberries. Purée the remaining strawberries in a food processor or blender and serve as a sauce with the roll.