Bostock is a French breakfast made from day-old brioche that is topped with jam and frangipane and baked in the oven. Frangipane is an almond cream used to fill tarts and pastries.
Combine the ingredients in a small pot. Heat the mixture on low to dissolve the sugar and help the rhubarb release some of its water. Increase the heat to medium–high and bring the mixture to a boil. Boil the jam until it is thick (the temperature will be between 101 ºC (215 *F) and 105 ºC (221 °F) when it's set). Take the jam off the heat and let cool completely.
To make the frangipane
In the bowl of an electric mixer fitted with the paddle attachment, cream together the butter, ground almonds, and the sugar. When the mixture resembles a thick, creamy paste, beat in the egg yolk, scraping down the sides of the bowl as needed. Add the orange blossom water, vanilla, and salt, and mix it in.
To assemble the bostock
Preheat the oven to 350 ºF. Place the 6 slices on a parchment-lined baking sheet. Divide the jam between the slices and smear it on evenly. Divide the frangipane between the slices, in dollops. Spread carefully with an offset spatula. This isn't easy to do so take your time to cover the jam and smear the frangipane from edge to edge. It will look rustic. Don't worry. Press 3 strips of rhubarb into the frangipane layer, then sprinkle with sliced almonds.
Bake the bostock in the preheated oven for about 25 to 30 minutes until the edges begin to brown and the rhubarb slices are soft.
Transfer the baking sheet to the top rack of the oven and switch the oven setting to broil and the temperature to 450 ºF (230 °C). Broil the bostock to lightly brown the tops (watch it so it doesn't catch fire or scorch too much).
Let cool slightly (or completely), then transfer the bostock to plates and serve warm or room temperature, dusted with powdered sugar.