To make the pastry: combine the flour, salt and sugar in a bowl. Cut in the cold butter with a pastry cutter or the back of a fork, or use a mixer. Avoid overmixing, so that you end up with larger chunks of butter than you would think. This will make the pastry more flaky once baked. Drizzle in the water and bring it all together without working the dough. Shape into a ball and wrap in cling film. Leave to rest in the fridge for about half an hour.
To make the frangipane: In the bowl of an electric mixer, beat the butter and sugar until pale and creamy. Add the ground almonds and beat well. Add the kirsch (if using), egg yolk and cream and beat well. Use while still soft.
To make the Rhubarb Galette: Preheat the oven to 220c/180c (fan)/gas 6. Line a laarge flat baking tray or pizza pan with parchment paper.
On a lightly floured surface, roll the pastry out into a large circle about 30cm diameter. Slide the pastry on to the lined baking tray, then cover and put back in the fridge or freezer to chill while you prepare the rhubarb.
Cut off and discard the rhubarb leaves, then trim off and reserve the end of each stalk. Cut the rhubarb into three or four pieces and then cut each piece lengthways into 5mm strips. Toss the rhubarb pieces with the sugar and the flour.
Remove the pastry from the fridge and arrange a ring of rhubarb pieces around the perimeter of the pastry, leaving a border of about 4cm. Roll the edge of the pastry up over the rhubarb to make a decorative edge. Spread a little soft frangipane on the centre of the pastry and pile the remaining rhubarb over this.
Brush the pastry with the melted butter and sprinkle with the remaining sugar. Bake in the centre of the oven for about 45 minutes. After about 15 minutes, rotate the baking tray and, using a spatula, push down the rhubarb to flatten it. Rotate again after 15 more minutes to ensure even baking.