Scrumptious steak & stout pie

INGREDIENTS

• 1 knob of unsalted butter
• 3 red onions, peeled and finely sliced
• 3 sprigs of fresh rosemary, leaves picked and finely chopped
• 3 sprigs of fresh thyme, leaves picked
• 3 fresh bay leaves
• 1 kg beef skirt
• 500 g chestnut mushrooms, cleaned and sliced
• 2 tablespoons tomato purée
• sea salt
• freshly ground black pepper
• 3 tablespoons balsamic vinegar
• 300 ml good-quality stout or dark ale
• 3 heaped tablespoons plain flour
• 750 ml hot organic beef stock
• 100 g Westcombe Cheddar, sliced
• 1 large free-range egg, beaten
• 300 g plain flour, plus extra for dusting
• 100 g Atora shredded suet
• 100 g unsalted cold butter, cubed