Remove the leaves from the rhubarb and discard. Wash the rhubarb, then cut each stalk down the middle lengthwise Chop into ~2″ pieces, chuck into a large bowl with the rest of the rhubarb filling ingredients. Toss to coat and leave to macerate.
Take a piece of parchment paper, cut it so that it fits a large cookie tray. Place the parchment onto your work surface and dust with flour. Unwrap the chilled pastry, dust with flour, and roll out into a 12 to 14″ circle (it doesn’t matter if it gets too large for the paper, you’ll fold it in later).
Pile the rhubarb filling mixture into the center of the pastry (leave as much of the juice behind as possible), spreading it out and leaving a 2″ border.
Fold the pastry in (brush with the leftover juice from the filling, to help it to stick together). Brush the pastry with buttermilk/milk/cream and sprinkle the whole thing with raw sugar. Dot with butter and chill for 30 minutes.
Bake in an oven preheated to 180 degrees for 35-45 minutes until golden brown and bubbling.