Preheat the oven to 200°C (180°C fan / 400°F). Line a large baking tray with baking parchment.
Very finely chop the pistachios (or pulse in a food processor). Place in a small bowl with the sugar and mix well.
In a separate small bowl, beat the egg until smooth.
Trim the tough ends and leaves off of the rhubarb. Cut each stalk into 20cm (8-inch) lengths.
Unroll the pastry and lay it out on a work surface. Trim to a 25cm (10-inch) square. Using the tip of a knife, make an incision all the way around the square about 2.5cm (1-inch) in from the outside edge. Do not cut all the way through. The rhubarb will sit within the resulting 'frame'. Transfer to a piece of baking parchment so you can move it around freely.
Cut four 22.5 x 2.5cm (9 x 1 inch) strips out of the remaining pastry. Brush the scored pastry 'frame' with the beaten egg, then lay the strips of pastry over it. You don't want the strips to overlap so place each strip flush against the next and continue all the way around to make a frame. If at any point the pastry feels warm and is hard to work with, transfer to the fridge to chill for 10 minutes. Brush the strips with more egg wash.
Prick the base all over with a fork then scatter three quarters of the pistachio mixture over it evenly.
Transfer the pastry to a baking tray and bake for 15 minutes. Remove from the oven and arrange the rhubarb as tightly and neatly as possible in a row inside the frame.
Scatter the remaining pistachio sugar haphazardly over the rhubarb and flick a little water over it. Bake for a further 15-20 minutes until the pastry is golden, the base is crisp, and the rhubarb tender.
To serve, brush the rhubarb with a little warmed honey (if using) and cut into portions. Add a dollop of creme fraiche or ice cream.