Preheat oven to 220C/425F and line a large baking sheet with baking paper.
Add oil or butter to a fry pan on medium-high heat. Add the onions, stir for 3-4 minutes until translucent. Add the mushrooms, stock and red wine and cook uncovered until reduced, about 10 minutes.
Place pastry sheets onto the lined baking sheet, join at the centre by overlapping the sheets and pressing down with the back of a fork. Pour the cooked mushroom mixture onto the centre. Spread the mushroom mixture out so it is evenly distributed. Fold the edges of the pastry over so you have a few centimetres as an edge. Bake for 25 minutes until golden brown.
Top with crumbled feta, rocket, balsamic glaze and sea salt flakes.