1. Cut the rhubarb stalks into 3-4cm lengths. Mix in a bowl with the brown sugar, vanilla seeds and pod. Set aside.
2. To make the pastry mix the flour, sugar and salt together in a large bowl. Rub in the butter until you have a sandy texture. Using a table knife, mix in the egg with 4-5 tbsp of ice-cold water, to form a rough dough. Knead very briefly until the dough comes together, then cut off one-third. Flatten each piece into a roundel about 2cm thick and wrap in clingfilm or parchment. Chill for at least 20 minutes, or for up to 1 hour.
3. Line a large baking tray with baking parchment. Dust a clean surface with flour. Roll the larger pastry sheet to a rough 30cm roundel, about the thickness of a £1 coin. Transfer to the prepared tray. Scatter the ground almonds into the centre and top with the rhubarb mixture, leaving a 5cm border exposed. Discard the vanilla pod.
4. Roll the smaller piece of pastry to a 23-25cm roundel and cut into 7 strips, each about 3cm wide. Lay four strips, evenly spaced, over the fruit. Working one at a time, lay the remaining 3 strips in the opposite direction, interweaving them with the other strips as you go, to create a lattice, then tucking the ends under neatly. Brush with beaten egg and scatter with demerara sugar. Chill for at least 30 minutes or freeze for 15 minutes.
5. Preheat the oven to 220°C, gas mark 7. Bake the galette towards the bottom of the oven for 15 minutes. Reduce the temperature to 190°C, gas mark 5, and cook for a final 15-20 minutes until golden and the base is crisp. Rest for at least 30 minutes, before scattering with toasted flaked almonds. Serve warm or cold. Delicious with vanilla ice cream.