Preheat your oven to 180°C (160°C fan forced) and line a baking tray with parchment paper.

Combine caster sugar, water, and orange juice in a saucepan and bring to a boil to make the sugar syrup.

Beat together softened unsalted butter and sugar until fluffy, then add egg, vanilla, orange zest, and almond meal to make the frangipane.

Place stale brioche slices on the baking tray and brush half of the sugar syrup over each slice before spreading almond frangipane on top.

Arrange sliced rhubarb on the frangipane and sprinkle with flaked almonds, then brush with remaining syrup.

Bake for approximately 40 minutes or until golden brown; cover with foil if they brown too quickly.

Remove from oven, let cool slightly, and dust with icing sugar before serving.