In a saucepan, heat the sugar and glucose to 163°C (325°F), stirring regularly. Pour the syrup onto a plate covered with greaseproof (wax) paper or onto a silicone pastry mat. When cooled, break it into the bowl of a blender and blend to a powder.

Preheat the oven to 150°C (300°F). Place four 10 × 6.5 cm (4 × 21⁄2 in) cookie cutters onto a baking sheet covered with a silicone pastry mat. Using a conical strainer, sprinkle the powder into the cookie cutters. Bake for 5–6 minutes, until translucent like glass. Remove the pastry mat to a work surface and leave to cool. Using a small spatula, loosen the squares of opaline.

Brush the costmary leaves with the beaten egg white. Sprinkle with icing (confectioners’) sugar and shake gently to remove any excess. Leave to dry at room temperature for 24 hours.

In a saucepan, combine the sugar with 400 g water and bring to the boil to make a syrup. Add the lemon juice and zest, lower the heat and cook for 5 minutes. Poach the rhubarb pieces in the syrup over a low heat until they begins to soften. Leave the rhubarb to cool in the syrup.

Preheat the oven to 180°C (350°F). In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter with the sugar for 1 minute until pale. Add the sifted flour and baking powder, then add the eggs one at a time.

Gently incorporate the rest of the ingredients using a spatula. On a baking sheet lined with greaseproof (wax) paper, spread the mixture out to a thickness of 3–4 mm (about 1/8 inch). Bake in the oven for 10 minutes.

Preheat the oven to 150°C (300°F). Bake the fouace/brioche slices in the oven for about 10 minutes, until lightly golden. Leave to cool, then reduce to powder in a blender. Set aside 60 g of this powder.

Soak the gelatin in a little cold water until soft, then squeeze dry. In a saucepan, bring the milk, butter, hazelnut praline and sugar to the boil. Stir in the gelatin, fouace powder and orange blossom water. Just before the mixture begins to set (at around 35°C/95°F), stir in the whipped cream. Set aside in the refrigerator.

Stir all the ingredients together until smooth.

Make a quenelle of fouace cream and place on a plate. Arrange the poached rhubarb on the plate and top with an opaline square. Sprinkle with some croustillant. Add some citron cream and crystallised costmary leaves. Finish with a thin slice of fresh rhubarb.