Pre-heat oven to 180°C. In a large pan, heat the olive oil and butter and fry the leeks, onions, garlic and tarragon for 10 minutes, or until softened. Add an extra drizzle of olive oil if necessary.
Add in the mushrooms along with an extra one tablespoon olive oil if necessary, and cook until the onions are tender and lightly golden.
Add in the stock, balsamic vinegar and cream along with a good pinch of salt and freshly ground pepper. Bring to a simmer, check for seasoning and transfer to a shallow baking dish (the mixture must fit just to the top of the dish so that the pastry sits snugly on top).
Place the pastry sheet over the top of the pie and use a fork to press down the edges all the way around. Trim off any excess pastry using a knife, make two small holes in the centre of the pie to allow steam to escape and bake for 30 minutes or until the pastry is golden.
Serve immediately with a simple side salad.
Recipe courtesy of Royal Baking Powder’s Sarah Graham