Start by washing and trimming the ends of the rhubarb.
Then using a 15cm bottomed tarte tatin dish (or oven proof frying pan) as a template, cut lengths of rhubarb to fit the base and make your pattern in there. If you want the Union Jack/cartwheel affect I went for then you start with one piece the length of the dish and then further shorter lengths to form the stripes, cutting the corner wise lengths into a point so that they can slot in and the cut shorter lengths to fit in around the circumference.
Once you have your rhubarb pattern mapped out in you dish then invert it quickly onto a board so that you can easily recreate it.
Melt the butter over a gentle heat in the dish.
Add the sugar and gently heat through for about five minutes until bubbling and starting to caramelise, tip and swirl occasionally, don't use a wooden spoon (don't panic that it looks lumpy or pale, you can't go wrong).
Take the dish off the heat and reassemble your rhubarb pattern.
Preheat the oven to 200°C.
Place back on a gentle heat and cook for another 10 minutes (during which time most or all of the sugar will dissolve).
Then fill in the gaps with the blueberries and scatter over the vanilla sugar.
Next unroll the pastry out and prick all over.
Place the dish on a wooden board and drape the pastry over it.
Use scissors to cut around the dish leaving a 2cm overhang.
Tuck and fold the pastry under itself down into the dish.
Place on the middle shelf of oven and bake for 25 minutes until risen and golden.
Once baked, run a palette knife around the outside or the pastry and quickly invert onto a serving dish.
Best served hot with ice cream.