Picnic Basket Pie

Name: Picnic Basket Pie
Ingredients: 600g/1lb 5 oz plain flour; 300g/10½ oz unsalted butter, at room temperature but not soft; salt; ½ tsp baking powder; 50ml/2 fl oz milk, to brush; 1 free-range egg, beaten, to glaze; 2 tbsp sunflower oil; 2 medium red onions, chopped; 2 garlic cloves, in whole cloves, skin on, lightly crushed under fork; 3 Romano red peppers, chopped; 1 large courgette, chopped; 1 aubergine, chopped into 1cm/½in cubes; salt and freshly ground black pepper; 100g/3½ oz couscous; 90g/3¼ oz sun-dried tomato paste; 250g/9 oz halloumi; 100g/3½ oz mozzarella, grated; small bunch fresh oregano; small bunch fresh basil; 100g/3½ oz caster sugar