Heat the oven to 200C/180C fan/gas 6. Put the sweet potatoes and oil on a large baking tray with some seasoning and toss. Roast for 20-30 mins until golden and tender. Leave to cool.
Tip the rice into a bowl with the spring onions, cranberries, dill, parsley, lemon zest and half the lemon juice. Add half the beaten egg, then mix well. Tip in the cooled sweet potatoes and stir gently so you don’t break up the potato too much.
Trim a quarter off one of the pastry blocks, then roll the rest of the block out on a lightly floured surface to the thickness of a £1 coin – it should be a rectangle about 15 x 45cm. Combine the goat’s cheese with the remaining lemon juice to loosen, then spread over the pastry rectangle, leaving a 2cm border. Spoon the rice filling on top, and press down firmly to make a smooth shape. Stick the trimmed quarter onto the second pastry block and roll out to a rectangle that’s the thickness of a £1 coin – it should be large enough to cover the whole coulibiac. Lift the pastry on and press the edges together. Trim the edges, then crimp or press with a fork to seal. Brush all over with the remaining beaten egg, then score a pattern on if you like using a sharp knife, and sprinkle over the seeds. Chill for at least 30 mins, or up to 24 hrs.
Heat the oven to 200C/180C fan/gas 6. Bake the coulibiac for 40 mins until golden and crisp. Serve in thick slices, with dollops of lingonberry or cranberry sauce and soured cream.