400g/14oz chestnut mushrooms, thickly sliced
2 garlic cloves, crushed
600g/1lb 5oz potatoes, peeled and chopped into bite-size pieces
1 red onion, roughly chopped
1 tsp chopped rosemary
salt and freshly ground black pepper
1 tbsp coconut oil, at room temperature (for brushing)
2 tbsp vegetable oil
320g ready-rolled shortcrust pastry (see recipe tip)