Heat oven to 220C/fan 200C/gas 7. Roll the pastry out thinly and, using a plate or cutter, cut out 4 circles about 14cm in
diameter. Use a slightly smaller cutter to cut a circle out of the middle and trim away the outside border, reserving these trimmings.
Lay the 4 circles on a baking sheet, prick all over with a fork and brush the edges with egg. Use the pastry trimmings to make a border around the edge like a vol-au-vent and press down gently with your thumb. Chill until needed. This can be done up to 1 day ahead or frozen for up to 1 month.
In a small bowl, mix the demerara sugar with the crème fraîche and spread a thin amount over the base of each tart. Neatly arrange slices of rhubarb, overlapping in a floral pattern, over the crème fraîche, then dust generously with icing sugar. Bake the tarts for 20 mins until puffed up and golden. Leave to rest for a few mins and eat warm with a scoop of ice cream, if you like.