Portobello Steak 'n' Kidney Bean Pie - Meat Free Monday

Ingredients

• 1 tablespoon vegetable oil
• 1 red onion, thinly sliced
• 3 garlic cloves, crushed
• 1 tablespoon balsamic vinegar
• 1 teaspoon yeast extract or miso paste
• 1 tablespoon tomato puree (paste)
• 300 g/10 oz portobello
• mushrooms, chopped into thick chunks
• 200 g/7 oz mixed mushrooms, such as shiitake or chestnut (cremini), sliced
• 2 tablespoons plain (all-purpose) flour
• 350 ml/12 fl oz (1 cup) vegetable stock
• 1 x 400 g/14 oz can kidney beans, drained and rinsed
• 2 x 320 g/11 oz sheets of vegan puff pastry
• aquafaba or unsweetened dairy free milk, for brushing