Preheat the oven to 200C/180C Fan/Gas 6. Line a large baking tray.
Start by making the pastry, if making your own. Do this by adding the flour and butter to a large bowl and rub together with your fingertips until roughly combined. You do not want to take it as far as breadcrumbs, there should still be bits of butter visible in places. Add the water and make into a ball. Wrap and chill for 30 mins, while making your sausage filling.
Add the red onions and sausages to the baking tray, drizzle in olive oil and bake until cooked (according to cooking instructions for the sausages).
Remove the pastry from the fridge, roll in one direction to form a long rectangle. Fold the top edge down to just below the middle. Then fold the bottom edge over this, to form a square with 3 layers (book end). Rotate this block 90 Degrees and repeat. Wrap and chill for another 20 minutes, whilst the sausages/onions are cooling.
Once cool, chop the sausages and onions using a sharp knife until in small chunks. Add to a bowl along with the brown sugar, mustard, salt, pepper and mixed herbs. Stir to combine, adjusting seasoning to taste.
Preheat the oven to 220C/200C Fan/Gas 7. Line two/three large baking trays.
Roll the pastry into a large rectangle, about 18×4 inches. Spread the sausage filling across the long edge, in a cylinder/log shape. Leave about an inch of pastry to one side, and more room on the other side.
Take the edge of the pastry and roll it over the filling, keeping it as tight as possible. Keep rolling the pastry until you reach the end and have a log of pastry with the filling all inside.
Cut the pastry log into about 12 sections, each about 1.5 inches long. Place on baking sheets, at least 2 inches apart.
Optional: brush over a little vegan milk on each piece. Sprinkle over extra herbs and some pepper. Score some small diagonal lines in each to allow air to escape.
Bake for 20 minutes, until the tops are golden brown. If any filling spills out, wait a few minutes once out of the oven before using the back of a spoon to push the filling back into the pastry. Serve still warm or let cool before boxing up to take to a picnic/bbq/lunch box!