Ruby's Picnic Pie

Ingredients

• 100g caster sugar
• 600g plain flour
• 1 tsp salt
• 1/2 tsp baking powder
• For the filling:
• Salt and pepper to taste
• 125ml boiling water
• 2 medium red onions, thinly sliced
• 2 large garlic cloves, thinly sliced
• 3 romano peppers, quartered and cored
• 1 medium aubergine, thinly sliced
• About 5 tablespoons olive oil
• 1 large courgette, thinly sliced
• 200g vegetarian halloumi
• 100g couscous
• 3 tablespoons sun-dried tomato paste
• Leaves from a small bunch each of fresh oregano and basil, chopped
• 100g vegetarian mozzarella, grated
• For the pastry:
• 300g unsalted butter, at cool room temperature, diced
• About 150ml icy cold water
• 3 tablespoons cold milk, for brushing
• Beaten egg, to glaze
• To finish: