Heat butter over a medium low heat in a frypan and add onion. Cook for 5 minutes or until softened, turning occasionally. Sprinkle over brown sugar and balsamic vinegar and cook for a further 3 minutes or until onions are glazed and caramelised. Set aside and allow to cool slightly.
Meanwhile, combine flour and salt in a large bowl and make a well in the centre. In a separate bowl, whisk together egg, olive oil and milk. Stir into dry ingredients until it forms a soft dough. Turn mixture onto a floured surface and knead lightly. Divide the dough into 2 and roll out into rough ovals. Place onto lightly oil oven trays.
Combine Leggo’s Tomato Paste and balsamic vinegar and spread ½ the mixture over each pizza base. Top with prosciutto and caramelised onion, drizzling with any remaining pan juices from the onions. Bake in a preheated oven at 180°C for 10 minutes or until base is cooked and prosciutto is crispy.
Top with mozzarella and allow the pizza to sit for 1-2 minutes or until cheese starts to melt. Garnish with parsley and serve.