Preheat the oven to 220C/425F/Gas 7. Line two 46cm x 33cm/18in x 13in baking trays with greaseproof paper. Grease 12 x 7cm/2¾in chefs' rings.
For the filling, place the rhubarb, strawberries, sugar and water into a small pan and simmer for 10-15 minutes or until the fruit is soft. Strain the cooking liquid through a fine sieve and reserve the cooked fruit. Set aside to cool for 15 minutes.
Return the fruit juices to the pan and boil until reduced to a syrup. Fold the cooked fruit through the syrup and set aside to cool.
Gently beat the double cream, cream cheese and mascarpone together in a bowl until well combined. Swirl through spoonfuls of the sauce and chill in fridge until required.
For the joconde-décor paste, cream the butter and sugar together in a bowl until light and fluffy. Gradually beat in the egg whites, then fold in the sifted flour and a few drops of rec food colouring.
Spoon the paste into a piping bag fitted with a 5mm/¼in plain nozzle and pipe a pattern onto the baking trays. Chill in the freezer.
To make the joconde sponge, whisk the egg whites until soft peaks form when the whisk is removed. Add the granulated sugar and continue whisking until stiff peaks are formed. Cover with cling film to prevent the meringue collapsing.
Beat the almonds, icing sugar and eggs in a bowl until the mixture is light and fluffy. Mix in the flour.
Gently fold in the meringue mixture using a large spatula. Mix a cupful of the sponge batter into the clarified butter, then fold this into the remaining batter.
Remove the baking trays with the decorated sheets of greaseproof paper from the freezer. Divide the batter evenly between the two baking trays, spreading it smoothly over the decorations and ensuring it is level.
Bake for 5-7 minutes or until the sponges are lightly browned. Cover each sponge with a sheet of greaseproof paper then upturn the baking trays onto the work surface and peel off the paper to reveal the pattern.
To assemble the cheesecakes, cut 16cm x 5cm/6¼in x 2in strips of the cooled sponge and use them to line the greased rings, ensuring the pattern is facing outwards. Cut circles of sponge slightly smaller than 5cm/2in and press into the bases of the rings. Pipe in the filling and level with a palette knife dipped in hot water. Chill in fridge for 30 minutes, then carefully remove the metal rings.