1: Begin by preparing the strawberry syrup: Put 300 g strawberries, cut into pieces, in a bowl which can be used in a bain-marie. Add 100 g caster sugar. Cover with stretch cooking film to seal the container well and place in a bain-marie over very low heat for one hour.
2: At the end of this time, you will have a delicious strawberry syrup. Strain through a sieve. The pieces of strawberry left will not have much flavour, so you can either eat them or throw them away. Note: French chefs call this "clarified strawberries", rather than a syrup. Set aside.
3: Prepare 350 g confectioner's custard (crème pâtissière, or french pastry cream). When it is made and has cooled a little (wait 15 minutes), incorporate 150 g butter very cold and cut into small pieces, beating thoroughly until the butter is well mixed in. This cream is a "crème mousseline".
4: Prepare 1 génoise (genoa sponge). When cooked and cooled, cut this to the shape of your cake, round or square. You can see that here I have cut it round using a dessert ring. If you do not have one, you can cut round a small plate laid on the sponge.
5: Split the genoa sponge in two through the middle.
6: Place one circle of sponge on the serving plate and place a dessert ring over it.
7: Soak the the top of this sponge circle with half the strawberry syrup.
8: Spread a thin layer of crème mousseline over the sponge.
9: Arrange straberries cut in half around the circle with the cut side towards the outside.
10: Then fill the circle with whole strawberries, placed upright.
11: Spread the remaining crème mousseline over the strawberries; ideally, they should be completely covered. Tap the plate gently on the worktop to settle the cream.
12: Lay the other circle of genoa sponge on a wire rack and soak with the remaining strawberry syrup, using a brush.
13: Then turn this over onto the dessert inside the ring. This is a tricky operation, so I advise you to turn it over smartly onto a baking sheet to (along with the rack), then slide it onto the top of the cake. Press gently on the top to make sure it is flat and even.
15: Decorate with a half strawberry and brush with an apricot glaze. This is not essential, but adds an attractive finish and prevents the marzipan from drying out.
16: At the last minute, gently remove the cake from the dessert ring. Your "fraisier" is ready to be served and enjoyed.
Remarks
If you do not have a dessert ring, use a circle of strong card, lined with a piece of stretch plastic film, held in place with a paper clip. For a quicker finish, you can simply sprinkle the top of the cake with icing sugar in place of the marzipan. For Gaston Lenôtre, a strawberry cake should be made with butter cream rather than mousseline, and he called it a "Bagatelle".