Vegetarian haggis

Name: Vegetarian haggis
Ingredients: 60g/1¼oz yellow split peas (soaked in boiling water for an hour, or in cold water overnight, then drained); 60g/1¼oz pearl barley; 60g/1¼oz butter, plus extra for greasing; 2 carrots, peeled and finely diced; 100g/3½oz chestnut mushrooms, finely chopped; 1 large onion, finely diced; 100g/3½oz porridge oats; ½ tsp ground coriander; 1 tsp ground black pepper, plus extra for seasoning; 1 tsp ground allspice; 500ml/18fl oz vegetable stock; 2 tbsp treacle; 1 tbsp yeast extract; salt and freshly ground black pepper