Place the freshly cut rhubarb in a large pan along with the sugar, butter and water. Heat gently and stir occasionally, bring to the boil then simmer for 5 minutes. Remove from the heat - the rhubarb should be soft but still holding its shape.
Strain the rhubarb and pour the remaining juices into a small sauce pan. Place the stewed rhubarb in a small bowl and leave to cool.
To make the jam
Add 75g of golden caster sugar to the saucepan containing the rhubarb juice. Heat gently for 5-10 minutes until it starts to bubble and thicken. Remove from the heat and set to one side.
To make the Pastry
In a food processor pulse the flour, ground almonds, butter, icing sugar and salt until resembling fine breadcrumbs. Alternatively rub together with your fingers.
Next place the egg yolk, vanilla extract and tbsp cold water into a jug and whisk together well. Gradually add the egg mix to the breadcrumbs whilst continuing to pulse and mix. Once all the egg is added the mixture should have come together into a pastry. Turn out onto a floured work surface and press together into a ball. Divide the pastry ball into 2 and wrap each in cling film and chill in the fridge for 20 minutes. (Cooling the pastry will prevent shrinkage during baking and will give a nice crumbly finish.)
To Make the Galette
Pre heat the oven to 180 degrees C, Fan. Prepare a large baking tray lined with greaseproof paper
After 20 minute remove one of the pastry balls. Unwrap and place on a floured work surface. Roll out the pastry into a roughly round shape, 1-2mm in thickness. Use the rolling pin the carefully transfer to your lined tray. Add half of the stewed rhubarb to the middle of the pastry, leaving a 3cm border around the edges. Slice up a few thin strips of apple, around 2.5cm in length. Place the apple in the middle of the pastry over the rhubarb - I displayed mine in a star shape. Roughly fold over the edges of the pastry to seal in the fruit but still leaving the middle open and visible.
Finally spoon some of the rhubarb jam over the fruit and brush the outer edges of pastry with whisked egg.
Place in the centre of the oven and bake for 25 minutes or until the pastry is golden brown.
Repeat this with the remaining pastry and rhubarb to make the second galette.
Slice and serve with a dusting of icing sugar and warm custard.