Preheat the oven to 350 degrees.

Warm the water and sugar in a medium non-reactive pot until the sugar has dissolved.

Wash and slice the rhubarb into about 4.5 inch pieces (or as long as your pieces of brioche toast are wide). Arrange them in a baking dish and pour the simple syrup on top. Place a piece of parchment or fig leaves over the dish and press it gently to the surface of the poaching liquid; this will help the rhubarb cook more evenly and prevent it from bobbing to the surface.

Place the dish in the oven and bake for about 20 minutes. The aim is to lightly cook the rhubarb,you don’t want it to get mushy. If the stalks are thin, you may want to check them at 10 or 15 minutes.

Remove the baking dish from the oven and allow the rhubarb to cool to room temperature in the poaching liquid. 5.In the bowl of a stand mixer (or with a handheld mixer), cream the butter, sugar, pistachios, and salt together until light and fluffy. Add the flour and egg white and beat until just combined.

When the rhubarb has cooled enough to handle, remove it from the poaching liquid. You can keep the stalks in thick bars, or cut them down into smaller pieces.

Brush the brioche with the poaching liquid, giving it a good soak. You will have liquid left over - save it for cocktails or lemonade!

Spread pistachio frangipane over the soaked brioche. Arrange the rhubarb decoratively on top, nestling it into the frangipane. Sprinkle a little granulated sugar on top.

Bake the bostock at 350 degrees for about 30-40 minutes, or until the edges of the bostock are quite crisp and the pistachio frangipane has just begun to brown.

The bostock may be eaten warm or at room temperature, and are best the day they are made. You can make both the poached rhubarb and the frangipane ahead of time - you’ll need to refrigerate the frangipane for up to three days, and let come to room temperature before using.